Humans and aliens have clashed more often than anyone can count. With alien monsters constantly finding their way to Earth, when two different species meet, especially in the realm of cinema, encounters typically aren’t of the benevolent sort. Yet somehow, earthlings pull back from the brink (usually) and win the day. In quoting the Vision in “Avengers: Age of Ultron,”, “Our very strength invites challenge, challenge incites conflict, and conflict breeds catastrophe.”

RELATED: 15 Superheroes You Forget Are Aliens

To that end, we’re taking a look at the most dangerous aliens humanity has encountered in movies. These are creatures from the stuff of nightmares, some of whom have little to no weaknesses and only through blind luck are defeated. With that in mind, here are the 20 most dangerous aliens to have found their way to Earth.

20 THEY LIVE!

they live

Aliens sure do love causing paranoia amongst the natives. In John Carpenter’s “They Live,” there is no alien invasion or massive battle for the fate of the Earth; the aliens are already here. No stronger or deadlier than us humans, their physical power isn’t the real threat. Rather, they’ve assimilated entirely into our way of life, taking up human disguises, and controlling the world’s population through cleverly constructed ads with subliminal messages. Nobody fired a single shot during this takeover. They’d already won and we never even knew it.

Only when a mysterious pair of sunglasses gives Roddy Piper the ability to see the aliens for what they are does humanity have a fighting chance. At film’s end, Roddy destroys the mask the aliens have built for themselves, revealing them for all the world to see. The only question is what do we do next? Do we accept domination or do we fight back?

19 POD PEOPLE

Invasion Of the Body Snatchers

Pod people has become a phrase synonymous with creepy aliens. Just like our friends from “They Live,” these interstellar creatures prefer subterfuge to fisticuffs. Deadly in every sense of the word, the pod people’s quiet invasion happens before anyone is wise to their game.

When alien plant spores crash on Earth (a pattern you’ll see more and more), they grow into large seedpods, each with the ability to reproduce a duplicate copy of a human they take. Once the pod reaches full development, they assimilate every physical characteristic, memory and personality trait of their hapless victim. Their only giveaway is they are devoid of any genuine emotion. It gets worse. If you fall asleep in the vicinity of their pods, they get you. It doesn’t matter how long or short your rest is; if you close your eyes, that’s it. The only way to escape is to flee large distances, far away from out of the fallout zone of the pods.

18 HUMANS (AVATAR)

Avatar

You probably weren’t expecting humans to fall on the list, but here we are. Humans, as seen in James Cameron’s “Avatar,” have descended on the peaceful planet of Pandora like a swarm of hungry locusts. Home to the peaceful Na’vi, the humans who are there are mining the planet, destroying anything and anyone in their path, in the pursuit of rare minerals.

As invasions go, this is pretty much what every alien who invades Earth is trying to accomplish. With the shoe on the other foot this time around, we see just how deadly humans can be. Armed with giant mech suits and more firepower than is reasonable, the Na’vi have only survived as long as they have on account of staying far away and minding their own business. Still, when the humans come knocking on their door, there is little the Na’vi can do to stop them. The human invasion force is led by Miles Quaritch, a ruthless colonel who relishes in the extermination of the Pandora's people. Only in teaming up with other humans, who are believers in their cause, do they emerge victorious, kicking man off the planet. Their victory should be considered short lived however, as there’s nothing to prevent humanity returning, this time with more weaponry and an axe to grind.

17 SLUG ALIENS (NIGHT OF THE CREEPS)

night of the creeps

You don’t usually see aliens fleeing other aliens, but that’s exactly what’s going on in “Night of the Creeps.” The year is 1959 and on board a spacecraft in the middle of the cold void of space, two aliens are running around trying to prevent a third alien from releasing an experiment. They fail and said alien experiment lands on earth.

Taking the form of a small slug monster, these critters hop from person to person, infecting them almost immediately and turning them into mindless zombies who crave flesh. While killing the slugs themselves might not be the hardest thing as you can just step on them, killing them en masse certainly is; encouraging drastic steps be taken, like flamethrowers. They are fast, sneaky and intelligent; once their infection spreads, it wouldn’t take long for the world to be quickly overrun. Only quick thinking allows for our heroes to prevail… or do they?

16 VENOM

Venom in Spider-Man 3.

Crash-landing on Earth, like so many evil aliens are wont to do, the venom symbiote crawls out of its crater and heads to the most powerful being it can find: Spider-Man. Augmenting Spidey’s strength and making him more vicious and out of control (especially on the dance floor) than ever before, the symbiote’s potential is downright frightening.

Once our webbed hero gets rid of the sentient black anger goo, it finds someone else: Eddie Brock. Giving the failed journalist all of Spider-Man’s powers and more, the creature feeds off Brock’s hatred, further boosting his strength. Only through chance is our hero able to discover the symbiote's weakness to sound (and fire, it should be noted). Should it ever cross the symbiote’s mind, it could easily splinter off its host and find someone new, letting the process repeat itself endlessly. In the comics, an accidental sliver of Venom is enough to give birth to Carnage; imagine if it wanted an army? But that’s a story for another time.

15 THE PREDATOR

Predator

Humanity has long thought itself an apex predator, suited to hunt and kill any thing. Humanity is wrong. Enter: Predator. Visiting our homeworld multiple times throughout history, Predators have proved time and time again just who is the best at what they do. Stocking their trophy rooms with the many skulls of their fallen prey, the ritualistic hunters have superhuman strength and an uncanny intelligence, the likes of which can outwit even the smartest warriors; the Predator never shirks away from a challenge.

When you want someone to go kill an Alien, who do you send? Not Signourney Weaver, but the Predators. More than capable of killing the bug-like monstrosities by the dozens, their technology and stratagems far outweigh anything humanity has to offer. Every encounter mankind has had with the Predator rarely ends well; if you’re able to gain their respect and survive an encounter, that means you’re probably pretty deadly too. Even so, we’re usually left thanking our lucky stars the killer species doesn’t decide to just come down and wipe us out. The fact that they could be behind you, waiting to remove your head from its spine, veiled in their vaunted cloaking technology, makes them all the more dangerous and fearsome.

14 IVAN OOZE

Ivan Ooze

While perhaps the goofiest alien on the list, Ivan Ooze shouldn’t be taken lightly. The central bad guy to the 1995 “Mighty Morphin Power Rangers: The Movie,” Ivan Ooze’s reign of terror begins nearly immediately as he’s freed from his weird purple egg... thing. You might laugh at the suggestion, but Ivan Ooze’s power is astonishing. In seconds, he defeats Rita Repulsa and Lord Zedd, villains who’ve given the Power Rangers grief for years, encompassing them in a small snow globe.

He then all but destroys Zordon and the morphin grid in just moments, depowering the Power Rangers in the process. On top of it all, he can create an endless supply of dangerous birds and giant robot monsters that he can then inhabit and control. Did we mention his ooze zombified the planet’s adult population, nearly getting them all to commit suicide? Zombifiying ooze. Think on that for a moment. If that’s not a guy with a lot of power, then who is?

13 THE CLOVERFIELD MONSTER

Cloverfield

More than capable of giving Godzilla a run for his money, once this creature falls to earth, all bets are off. Thought to have been sent as the first wave of an invasion force, the Cloverfield monster is as big as a skyscraper and seemingly impervious to most artillery. The monster might not be the brightest bulb in the shed, but with its ridiculous strength and durability, it doesn’t really matter.

“Cloverfield” does a great job in answering the question what would happen if a terrifying alien monster decided to claim New York City as its own; the answer, of course, being lots and lots of carnage, involving many a citizen being eaten or stepped on. Just being in close proximity to the creature is dangerous, what with the “small” alien mites that fall off its humongous frame, attacking anything that moves. Should one of them wound you, you can expect a speedy rate of infection that ultimately results in an explosive death.

12 SPACE GODZILLA

Spacegodzilla

The nightmare reflection to everything that defines Godzilla, Space Godzilla is one of the most dangerous interstellar creatures to find its way to our planet. A planet buster if ever there was one, his fusion of Godzilla cells with that of intense radiation from a black hole make him among the most fearsome enemies Godzilla has ever had to contend with.

Defeating Godzilla in battle multiple times -- something few kaiju can attest to -- along with MOGERA, a new mecha built by mankind to surpass Mechagodzilla, it takes the combined might of MOGERA and the King of the Monsters to put him down. Even then, it’s by just the skin of their teeth, assisted by blind luck. With seemingly limitless energy, Spacegodzilla siphoning energy from the planet’s core and from the outer reaches of space, he’s a beast with incalculable power and it’s only by severing his connection to said power that he becomes vulnerable.

11 KING GHIDORAH

King Ghidoarah

A legendary monster if ever there was one, and one of the most iconic creatures in kaiju lore, King Ghidoarh has the distinction of being Godzilla’s arch nemesis. Known throughout the galaxy for the destruction he leaves in his wake, King Ghidorah has laid waste to entire civilizations. An unholy force of destruction, the three-headed space dragon never faced opposition until he ran into a certain radioactive dinosaur we all know and love.

Rest easy, for despite his immense power, Ghidorah can be stopped, at least if Godzilla is around. Even then, Ghidorah’s might is such that it ftentimes requires a team of monsters to put down the winged beast. Only on one occasion has Godzilla successfully defeated Ghidoarah by himself. Armed with destructive gravity rays shooting out of his mouths and standing at nearly 500 feet (when he’s not flying at supersonic speeds), King Ghidorah’s mythic power can be withstood by only a fair few. Humanity, however, is not one of them.

10 INDEPENDENCE DAY ALIENS

Indpendence Day

Here we have yet another bunch of aliens who decide to fly in and take over the world. Wasting two perfectly good opportunities, their failed attempts don’t make them any less dangerous. Having previously razed world after world, all in the name searching for minerals so that they move on to the next victimized planet, Earth has been the only planet to successfully ward of their attacks.

Capable of wiping out entire cities in seconds, the Independence Day aliens’ power is truly awesome. Inside and outside of their crafts they make for formidable enemies, and it doesn’t hurt that they nearly succeed where many other failed in conquering the earth. They would’ve won too, despite all of humanity pulling together in one large team-up, if their one weakness wasn’t exploited: digital viruses. Though smart, these aliens are not Smart, and get fooled pretty often; their arrogance proves to be their downfall.

9 DECEPTICONS

Megatron from Transformers

Constantly at odds with Optimus Prime and the Autobots, the Decepticons have made their presence known to humans in not so quiet ways all throughout our history. Their goals have sometimes been mysterious, while other times ranging from questing for the All Spark to obtain limitless power, to just wanting to rule/destroy the planet. The Decepticons are a nearly unparalleled force in their universe.

Granted, they can be defeated, but their raw power makes them more than capable of taking on entire armies. Their ability to blend into our environment, taking the form of virtually any vehicle, makes them difficult to track and allows them the element of surprise. Alas, despite having battle ships the size of worlds, and the ability to transform and get close to whatever or whomever they need, their one major weakness seems constantly to be meddling teenagers and their best friend, who also happens to be giant robots.

8 XENOMORPHS

Alien

Arguably one of the deadliest movie monsters in history, Ridley Scott’s Aliens are not only terrifying, but also downright dangerous. Capable of breeding at an astronomical rate, turning any living-breathing organism into one more host, one more soldier for their army, the Aliens are born virtually ready for battle, becoming a type of shock troops, as it were. Should they be left unchecked, the Aliens are capable of overrunning the entire galaxy, like a horde of giant, killer cockroaches.

Able to adapt to virtually any climate or atmosphere, these monstrosities are strong, vicious and incredibly sneaky. The perfect killing organism, they don’t know fear or compassion. They blend into the shadows and attack when least expected. You’d be afraid to even wound one, as their acid blood will rain on you, baptizing you in the pain of an awful death. With few weakness and even fewer individuals capable of withstanding their assault, the Aliens are the stuff nightmares have nightmares about.

7 MARTIANS (WAR OF THE WORLDS)

War of The Worlds

In H.G. Wells’ “The War of The Worlds,” and in this case the two movie versions, Martians invade the Earth. It’s not even so much as a battle as an outright takeover, as the aliens and their crafts are impervious to even nuclear weapons. With nothing left to do except hide, humans quickly find themselves on the verge of extinction.

The Martians’ power is unmatched by anything the people of Earth have. There are no little tricks to beat them, they can’t be defeated through brute force, and overall, while they’re in their space crafts, they are virtually impervious to anything we can throw at them. Their one weakness, discovered by luck and timing isn’t anything human beings can take credit for; instead, this victory goes to the planet itself. The microbes in the air, the germs we breathe, end up being the Martians’ undoing; perhaps they should have done an environmental scan before they tried taking over our world.

6 GENERAL ZOD

General Zod

The light to Superman’s dark, General Zod represents the opposite of everything the Man of Steel stands for. Should Superman ever decide to turn on the people of Earth, the result would be very similar to if Zod ruled the planet. Every ounce the Kryptonian as his foe Kal-El, Zod has all of Superman’s powers, including super speed, super strength, flight, heat vision, hurricane breath… all the classics. Unlike Superman however, Zod shows none of the restraint or caring our hero has adopted for the people of Earth.

Whether portrayed by Terrence Stamp in "Superman II" or Michael Shannon in "Man of Steel," Zod’s characteristics are unchanging, swearing vengeance on Superman and willing (and more than capable) to laying waste to the entire planet in the name of his revenge. Should Superman fall, there’s little the people of Earth could to halt the angry Kryptonian from committing wholesale genocide. Hopefully someone’s been stockpiling a treasure trove of kryptonite somewhere.

5 THE THING

The Thing

It’s one thing if you can identify your opponent, but it’s another if your enemy can take the shape of anyone it comes into contact with, passing itself off as friend, family, or even you! The Thing, as seen in John Carpenter’s “The Thing” and the original “The Thing From Another World,” crash-landed on our planet eons ago, freezing itself in the ice and waiting until unsuspecting fools released it from its hibernation. In every instance, it remains unclear whether the Thing is trying to build a craft to take it back into space or a populated continent so that it can dominate our world, but one thing is for sure, this creature is as deadly as they come.

A shapeshifter with the ability to perfectly mimic any other creature it comes across, the moment it touches you, you’ve become infected, falling into its ranks. Should subterfuge fail, the Thing can easily rip apart its foe with its Lovecraftian tentacles and rows or razor-sharp teeth. Capable of spreading like a parasite, its rate of infection would be enough to consume the entire world. The fact that it can be killed almost doesn’t matter, as the paranoia it creates is enough to consume even the most stoic of individuals.

4 THE BLOB

The Blob

Crashing to Earth (hopefully you’re aware of a pattern by now) back in 1952 (and then again in 1988), this gelatinous life form began its rain of carnage quickly and without remorse, killing everything in sight. Seemingly unthinking, the Blob’s rampage was nothing short of legendary as it followed through with only one thing on its mind: consumption. Unstoppable in every sense of the word, nothing can injure it.

From bullets to electricity, it is seemingly impervious to pretty much anything humanity can throw at it. The more people it consumed, the larger the Blob became, until eventually it was the size of a building. With the potential to wipe out all life on the planet, our entire military arsenal, including even Steve McQueen, were unable to put a dent in the creature. Its only weakness? Being frozen and taken to the Arctic where it will remain trapped, “…as long as the Arctic remains cold.” Even then, the Blob isn’t dead. More than likely, it can’t be killed.

3 CALVIN

Calvin

The newest addition to the threat of intergalactic aliens is one of the most dangerous and nasty creatures ever encountered. Calvin, as seen in “Life,” starts as nothing more than a tiny little cell, having been frozen for millions of years on Mars’ surface. Unfortunately for the poor souls on the space station that Calvin gets shipped to by way of space probe, and all of humanity down on the planet below, it doesn’t take much for the little critter to come alive and start generating chaos.

While not inherently evil, Calvin is fueled by an insatiable desire to devour and grow, all in the name of its own survival; humans just happen to be a nearby food source. Calvin is a creature with human-level intelligence. Burrowing into and/or devouring the bodies of its victims, Calvin absorbs anyone it comes across, become stronger and bigger with every kill. All but indestructible, its one and only weakness is starving it of oxygen. Unfortunately, in escaping to Earth, a planet ripe with air, Calvin presumably will never be stopped, killing every sentient being along the way.

2 GALACTUS

Galactus

Arguably one of the most powerful entities on any list when discussing power sets, Galactus’ title the Devourer of Worlds isn’t just a cute nickname his friends came up with at a party. The last of his kind after surviving the Big Bang, Galactus now hungers constantly for sustenance. Unfortunately for the universe, he gains said sustenance by eating planets. Love it or hate it, the amorphous cloud that is Galactus in “Fantastic Four: Rise of the Silver Surfer,” may not share the distinguishing features of his comic book counterpart, but the scale of his power is the same, if not even more visually massive.

Capable of devouring entire planets with ease, there is no earthly weapon that can possibly harm him, much less stop him. It helps to have the Fantastic Four on our side, because you never know what Reed Richards will whip up, but all we can really do is hope that the next time Galactus comes around, the Silver Surfer will be there to stop him once more.

1 KLAATU AND GORT (THE DAY THE EARTH STOOD STILL)

Day The Earth Stood Still

Not all aliens are evil or hungry; some are purely neutral, even bordering the line of benevolence. Yet occasionally, despite their good intentions, they are a threat worse than anything you can imagine. Introducing our final entry: the dynamic duo of Klaatu and Gort. One is an alien who looks remarkably human; the other is his invincible android companion. The two come as a packaged deal and have arrived on Earth on behalf of an alien species that has been watching mankind. They come with a message, saying we must change our ways, stop being horrible to one another, and be the best we can be to turn our species into a proud and united one. Should we fail, Gort will do his duty and destroy the planet.

This is an adversary with no weaknesses and no way to be overcome. In both versions of “The Day The Earth Stood Still,” it’s more than implied there isn’t anything mankind has that can even offer a threat to Gort. The android can’t be starved of air, absorbed, frozen and is indestructible to any weapons; capable of surviving the annihilation of worlds. With incalculable power at its disposal, even taking Klaatu hostage would, and does, prove ineffective as that only spurns on Gort to eradicate the planet. Only by convincing the two that there is goodness in humankind do Klaatu and Gort leave; but they’ll be back. They can’t be stopped and they won’t be stopped in making the universe a better place.

Is there any extraterrestrial threat that you think is more dangerous than these aliens? Let us know in the comments!