The Avengers are Marvel's premiere superhero team. Made up of heroes from all over the world and beyond, the team regularly faces challenges too big for a single hero. From reality-warping madmen to galactic conquerors bent on destroying the Earth, the Avengers have faced down some of the most powerful villains in the multiverse. Fortunately, the team usually brings along some heavy hitters themselves. From Thor to Hulk, there are Avengers who can stand up to the most powerful of villains.
However, even with some of the most powerful heroes on their side, sometimes they come up against threats that are still too much for them. Some of their most powerful foes require the help of other heroes and teams like the X-Men and the Fantastic Four. Still others manage to defeat all those teams, and are often only defeated by their own hubris or are simply never defeated. A fair few of those who are never defeated are heroes themselves, or become heroes after proving their worth to the Avengers. Some of these entries take place in alternate universes, but most are the real Avengers getting defeated by real villains and heroes, from Thanos to Squirrel Girl, and Ultron to Punisher.
15 BLACK WIDOW (YELENA BELOVA)
In 1999, a new Black Widow was introduced. Just like Natasha Romanov, she was trained in the Red Room to be a Russian spy. However, she remains loyal to Russia for a time, until Natasha breaks her mental conditioning and encourages her to find her own identity separate from Russia. While she initially retires from spying, she is lured back by S.H.I.E.L.D.
Her brief return ends in disaster, and she is badly injured. Rescued and recruited by Hydra, she is given the powers of the Super-Adaptoid. She is able to fight and defeat much of the New Avengers, consisting of Luke Cage, Iron Man, Spider-Man, Wolverine, Spider-Woman, Ms. Marvel, and Sentry by absorbing their powers. She is eventually defeated by all 49 of Tony Stark's Iron Man armors, and by absorbing the powers of the Void from the Sentry.
Ultron is without question one of the Avengers' greatest foes. Created by Hank Pym, Ultron has been continually been one of the strongest foes the Avengers face, going back to the 1960s. Because Ultron has been around so long, he's racked up an impressive number of Avengers defeats. He first defeated the Avengers when he created his adamantium body that quickly became his iconic design.
Later, he defeats the Avengers, the Fantastic Four, and a large group of Inhumans, only to be stopped by the reality-warping powers of Franklin Richards. He would face the Avengers again several times over the next 40 years, sometimes winning, sometimes losing. Most impressively, in Age of Ultron (the comic event, not the movie), he defeats the Avengers so thoroughly, they have to break time in order to save the universe.
13 SQUIRREL GIRL
Squirrel Girl, a mutant with the power to speak to squirrels (along with various other squirrel-based abilities), is infamous for punching way above her weight. Of course, because this is usually a joke, she does it off screen. Among her defeats include Thanos, Doctor Doom, and Wolverine.
In The Unbeatable Squirrel Girl, Squirrel Girl finally got her own solo series. Eventually, she comes into conflict with a small Avengers team consisting of Falcon, Captain America, Spider-Man, and Black Widow. And in true Avengers fashion, she defeats them off-screen, apparently in a mere 26 seconds. Around the same time, Squirrel Girl would join another team of Avengers under the X-Men's Roberto Da Costa, and put her talents to use fighting villains like the Maker and a guy who totally wasn't Cthulu.
Punisher is a simple man. He hates criminals, so he shoots them with guns. This deep-seated hatred is borne out of the killing of his family in the crossfire a mob shootout. In Punisher Kills the Marvel Universe, Garth Ennis asks the question: What if Punisher's family had been killed in the crossfire of a superhero battle?
The answer becomes readily apparent as "bad things happen." Punisher starts by killing Cyclops, Hawkeye, and Shadowcat, then continues to work his way through the Earth-based Marvel heroes. Hired by a rich businessman disfigured by superhero crossfire, he manages to kill every hero in Marvel, including the Avengers. Eventually, at the end of it all (spoilers), after killing the last hero, Daredevil, Frank realizes there's one more kill to make: himself.
One of the often forgotten aspects of Thor's character is his occasional connection to a human. First was Donald Blake, and later was Jake Olson. While merged with Jake Olson, Thor was badly injured and needed to be separated by Odin in order to fully recover. However, before Thor could recover and be re-merged, Odin died in battle with Surtur and could not re-merge them.
Separated from a human influence, Thor, now king of Asgard, became increasingly more aggressive and less empathetic to humanity. Ultimately, Thor sought to restore the Asgardian gods to beings of worship and attempted to overthrow a country in Eastern Europe. This brought him into conflict with the Avengers. Upon their defeat, he conquered the world and ruled it for nearly 200 years. He eventually softened and traveled back in time to re-merge his younger self with Jake Olson, to prevent the tragedy.
In the lead-up to the infamous event Civil War, Hulk was launched into space by the Illuminati, a group of the most influential superheroes on the planet. Unfortunately for them, things didn't quite go to plan, leading Hulk to crash land on a different planet. He got involved in slave gladiator combat, eventually led a revolution, and became the ruler of the planet (you may recognize a bit of this from Thor: Ragnarok).
Along the way, he fell in love and got married. But tragedy struck when the ship he was sent in exploded, destroying the planet and killing his wife. He returned to Earth, madder than ever. He then easily defeated almost every hero on the planet, including two teams of Avengers, the Fantastic Four, the X-Men, and the Inhumans. He is eventually defeated by a series of weaponized Stark satellites returning him to his Bruce Banner form.
Taskmaster is an interesting character. Rarely a villain in his own right, he instead works as a mercenary and villain trainer. His powers allow him to perfectly mimic any fighting style he sees, and he puts these skills to great use against the Avengers in his first appearance. At first, he used his powers to train henchmen and thugs. He was discovered when his clone sent word to the Avengers asking for help.
He manages to capture Yellowjacket, the Wasp, and Ant-man rather easily when the Avengers attack his facility. He also manages to hold his own against both Captain America and Iron Man at the same time. However, he is forced to flee when faced with Jocasta, Ultron's newly created robot bride. Due to his inexperience with her fighting style and abilities, he was unable to predict her next move.
POD is a recent creation, first appearing in Jonathan Hickman's Avengers. Standing for Planetary Operational Defense (System), the POD armor was designed as a defense mechanism against the impending threat of the Builders. Along with an array of standard flying brick powers, POD could also adapt to and exceed to whatever power was brought against it.
With this ability, POD was able to handily defeat the rather large Avengers team arrayed against it, including heavy hitters like Thor, Hyperion, Captain Universe, Captain Marvel, and Sunspot. It was never truly defeated, but captured by the villain Superia by trapping it between dimensions. Eventually, POD is rescued, and joins Roberto Da Costa's New Avengers team, where it operated as one of the most powerful members before a devastating battle led to a large scaling back of its powers.
7 ADAM WARLOCK
Before Thanos got a hold of the Infinity Gems and Gauntlet, the Soul Gem belonged to Adam Warlock. Adam Warlock, if you're wondering, was created to be the perfect human being and the pinnacle of future human evolution. He eventually wound up in space and opposed Thanos, who sought the power of the Infinity Gems.
After the events of Infinity Gauntlet, Adam sets up the Infinity Watch to protect the Gems against those who would misuse them. The Infinity Watch frequently found itself working with Thanos to prevent others from using the gems, including Warlock's evil half, Magus, and his good half, Goddess. Eventually, the Watch found itself attacked by the Avengers. Although Warlock was able to defeat them, he discovered they had been brainwashed by one of his old enemies.
6 SENTRY/THE VOID
The Sentry is one of the most powerful heroes in the Marvel universe. While he started off as a Superman knockoff, he eventually grew into a high-level reality warper with an evil half even more powerful than him. For every good action the Sentry took, the Void would make another, equal, evil action. While the Sentry wanted to good, the Void was deemed too dangerous and he chose to seal it away.
However, the Void eventually broke free. He battled the Avengers, defeating most of them, but with the help of Emma Frost, Sentry was able to seal him away again. Unfortunately, it never quite seemed to stick. The Void would battle and defeat the Avengers again during the Siege of Asgard. Although this battle ended with Sentry's death, a resurrected Sentry would face off against the Avengers Unity Squad as a Horseman of Apocalypse, defeating Thor and the rest of the Avengers before being convinced to aid them in a battle against a Celestial.
Like the Punisher, Deadpool Kills the Marvel Universe asks the question: What if Deadpool just killed everybody? In it, Deadpool is driven insane (well, more insane) by Psycho-Man, and begins a killing spree of every hero and villain in the Marvel universe. Starting with the Fantastic Four, Deadpool then uses Reed Richards' technology to begin his spree.
When he comes up against the Avengers, instead of facing them in open battle, he uses Pym Particles stolen from a murdered Hank Pym to blow up Avengers Tower, killing most of the Avengers inside. The survivors, Luke Cage and Thor, then face Deadpool directly. Deadpool reveals that he used the Pym Particles to plant miniaturized explosives in Cage's coffee, blowing him from the inside and bypassing Cage's unbreakable skin. Thor is killed by the same Pym Particles, this time making Mjolnir gigantic as it returns to his hand, crushing him.
4 GHOST RIDER
Way back in the early 1980s, Ghost Rider was a bit of a jerk. And in Avengers #214, he was very pissed off. After being cut off by X-Man Angel in a sports car, Johnny Blaze becomes the Ghost Rider and starts a reign of terror in a backwater town in Mexico. The Avengers, fresh off Hank Pym's infamous beating of Janet Van Dyne, are called in to deal with the situation.
The thing about Ghost Rider is that he's VERY powerful. While the Avengers at this time are pretty small (only Captain America, Thor, Tigra, and Iron Man show up to fight him), Ghost Rider single-handedly defeats Cap and Tigra easily. He also leaps up at Iron Man mid-flight, and blasts hellfire straight through Tony's helmet. Perhaps the most impressive defeat is Thor's. After outrunning Mjolnir, Ghost Rider then grabs it as it returns to Thor, and uses the momentum to bash Thor in the face with his motorcycle.
Cyclops is the rightest man to ever exist in the Marvel Universe. In Avengers Vs. X-Men, he winds up possessed by the Phoenix Force, as several X-Men tend to do. Together with four other X-Men also possessed by the Phoenix Force, he manages to conquer the planet and send the Avengers into hiding. Eventually, though, the other four Phoenix-powered X-Men are defeated, and their portion of the Phoenix Force goes to Cyclops.
In the final confrontation, Cyclops, now fully possessed by the Phoenix Force, loses control just as Jean Grey did and becomes Dark Phoenix. He defeats a combined team of Avengers and X-Men, killing Charles Xavier in the process. Eventually he is defeated by a bit of a nonsense deus ex machina in the form of Hope Summers and the Scarlet Witch. While he is defeated himself, his ultimate goal of rekindling the mutant population is fulfilled by Hope before the Phoenix is wished away.
Korvac is a cyborg from another universe with the ability to analyze and absorb any type of ability or energy. He uses this ability to first acquire the cosmic powers of the Grandmaster, and later the Power Cosmic of Galactus. In doing so, he becomes a god-like being capable of reshaping reality on a whim.
Eventually, Korvac finds himself brought into conflict with the Avengers during the Korvac Saga. A massive team of heroes, including the Avengers and the Guardians of the Galaxy, confront him, only to be defeated and killed in the battle. Eventually, Korvac, having killed nearly every hero that faced him, is weakened and commits suicide. In his final act, he restores the heroes he killed to life. Korvac occasionally returns, but at power levels far below his original battle with the Avengers.
Perhaps the most famous defeater of the Avengers on this list, Thanos is another of the Avengers' more dogged foes. Even without the Infinity Gauntlet, Thanos has proven himself to be a hugely powerful threat, often taking on entire teams of Avengers at once. However, it is with the Infinity Gauntlet that he can most consistently defeat the Avengers.
In Infinity Gauntlet, he starts his battle with a snap of his fingers, instantly erasing half of all life in the universe, including several Avengers. The Avengers take the battle to him, but are summarily defeated by Thanos's god-like might. Eventually, it is not the Avengers, but Thanos's own granddaughter Nebula who defeats the Gauntlet. Thanos agrees to assist the heroes in defeating the now power-mad Nebula, but commits suicide (0r so we think) to avoid capture after failing to regain the Gauntlet.