The Marvel universe, both in the comic books and the films, is highly populated and diverse. There is no shortage of alien races (both friendly and hostile) who also have their own versions of superheroes or supervillains. They may not be as clear-cut as Earth's, however, since let's face it, humans are simply weaker compared to other alien races in the Marvel universe.
How humans managed to exist and survive as long as they have is a miracle what with all the cosmic threats abound. Really, the Earth is at the mercy of the Avengers and mightier alien races which luckily don't see humans as prey or slaves. There are certain alien species in the Marvel universe that make us look like bacteria by comparison, here are 10 of them to remind humans not to get too uppity.
10 THE SOVEREIGN
Power is proportionate to arrogance when it comes to alien races in Marvel. It's similar to how we human beings think we're on top of the food chain or something. The Sovereign, a race of golden-skinned humanoid aliens, is no different. They are proud, xenophobic, and easily offended. They're known for their hubris and they have the technology to back that up.
Moreover, the Sovereign think themselves above biological conventions. They have no need for sexual reproduction since their technology allows them to control the DNA of their individuals through birthing pods. As a result, no individual is a waste of skin in the Sovereign and everyone has a predetermined role in their society; this leads to efficiency in any field. In case you missed them, they have been featured in MCU films like The Guardians of the Galaxy Vol. 2.
The Skrulls, while not as technologically impressive as some of the alien races higher up in this list are certified menaces of the Marvel universe. They are a race of alien shapeshifters and can imitate the appearance of others. This makes them highly effective and dangerous at espionage, assassinations, and other business that are usually preludes to war.
They have caused a lot of trouble for many planets and alien civilizations. Even a whole arc in the Avengers was dedicated to the Skrulls and their Secret Invasion. More than that, they are the sworn enemies of the Kree and have had many wars with them, each lasting quite long. These guys know how to take what they want and have carved themselves an empire among already established ones.
As mentioned above, they are the nemesis alien race of the Skrulls. The two wage long wars against each other from time to time; nevertheless, it's actually the Kree which has the upper hand in military and technology. It's not just warfare the Kree excels in. They also love to dabble in some genetic experimentation and were actually responsible for creating the Inhumans.
Additionally, they know how to create their own versions of Ultron without failing the same way humans did. The Kree managed to invent big autonomous robots known as the Kree Sentry which last for thousands of years and can fight for them. Last but not least, they have Ronan the Accuser and Mar-Vel (the guy responsible for making Carol Danvers Captain Marvel).
One of the biggest alien empires in Marvel, the Shi'ar are not as exclusive a race as the Sovereign. Originally, they were avian-descended humanoid aliens from their homeworld of Chandilar. However, they have grown to be welcoming to other species as well. Apart from being technologically advanced, they also spare no expense in their military capability. They used such means to conquer multiple worlds and multiple galaxies; hence forming the Shi'ar empire from the initial planet of Chandilar.
What makes them quite an intimidating empire is their Imperial Guard. They are the elite and personal bodyguards of the Shi'ar and somewhat an equivalent of the Avengers. One of the members of the Imperial Guard is a human named Isabel Kane, who also sidelined as an Avenger from time to time. The Shi'ar have actually conquered the Kree at one time. They also know how to defeat the Phoenix Force, one of the most powerful entities in the Marvel universe.
You'll certainly want for nothing else if you live in an extradimensional pocket of the universe away from all the wars of the Kree, the Skrulls, and the Shi'ar. That's what makes Asgardians (humanoid godly "aliens") so radiant. However, despite their nice sunny paradise of Asgard, they are no slouch when it comes to defending their home.
They actually have some of the biggest heavy-hitters in Marvel namely Thor, Odin, Hela, Loki, etc. and the means to travel around different dimensions. Against the Shi'ar, it will be a pretty close fight but the Asgardians might eventually end up winning. They are pretty much extradimensional mini-gods battling a conventional alien race.
5 THE WATCHERS
While the relatively younger species of the universe are content on waging wars and forging their own legacies, one of the oldest alien races ever just wants to record stuff. They are called The Watchers. They value knowledge more than anything. Still, they are rational and responsible about the wisdom they acquire; they actually refrain from interfering or helping another alien race ever since one species destroyed itself after receiving knowledge from them.
As such, the Watchers specialize in staying true to their name, preferring to watch instead. They wield tremendous power through their knowledge of the universe but refrain from using it for their own gain. It wouldn't really be fair to pit them against races who have their own military and technological forces, but the Watchers deserve to be high up on this list for their potential and their defiance against the Celestials.
They're not exactly just one species, but rather, a collection of immortal beings who have banded together. Some of them are the last of their race and existed before the Big Bang. In that regard, they are also among the oldest beings in the Marvel universe. Being immortal, there's really not much to do other than their hobbies.
One of them, Ego (portrayed as a planetary Celestial in Guardians of the Galaxy Vol. 2) loves to create stuff while another, the Grandmaster (not to be confused with Jeff Goldblum's Grandmaster in Thor: Ragnarok) loves to create games for mere mortals. Each Elder respects another and regard other Elders as brothers or sisters, though they often don't associate with one another.
If you thought the Asgardians were godly enough, then you haven't seen the Celestials in action. They are among the oldest and most powerful beings in the universe and like their creator, First Firmament (the first sentient cosmos, an abstract being), they can create life. They also can create death and some wanted to toy with life (through evolution, change, and death). This angered the First Firmament and it resulted in a war between the loyal Celestials (called Aspirants) and the rebellious ones. The First Firmament and the Aspirants lost and fled to another reality.
This left the rebellious Celestials with a reality of their own to do as they please with it. Somehow, they chose to interfere with human life and caused all sorts of evolutionary and cosmic anomalies there, giving birth to superheroes and mutants. Oh, they're also seemingly immortal and their cosmic powers are vast and beyond measure-- probably even without limits.
2 THE INFINITES
As powerful as Celestials may be, they are still somewhat made of organic stuff (they have skulls and brains), meaning they are physical beings. That means they can still be vanquished and a lot of them have been in the Marvel comics. The Infinites one the other hand, are abstract beings who also wield incalculable power and can easily traverse through several multiverses.
While they are abstract, they have shown their hands to be bigger than that of the anthropomorphic manifestation of Eternity (the embodiment of reality, another abstract being). With that, one of the Infinites was able to easily defeat Eternity... yes, some abstract being in Marvel defeated the multiverse itself. Meanwhile, the Celestials struggled to even defeat the First Firmament (just one universe). No one knows much about the Infinites, however, or the extent of their power or domain.
Their name explains what they are. The Beyonders don't exist anywhere in the multiverse. They are from a dimension outside of the multiverse and are near omnipotent. For that matter, no one has seen the Beyonders. These aliens only send their agents to transact with mere denizens of the multiverse. To give you an idea of their vastness, they can collect planets as part of their trinket museum.
The Beyonders also love to experiment and toy with the multiverse as if it was just an aquarium. At one time, they managed to kill all the Celestials along with other abstract beings like Eternity and Infinity. That was all for the sake of experimentation. They aren't invincible though; Doctor Doom was able to steal the powers of one Beyonder and literally became a god of the multiverse. Still, there is no doubt they are the most powerful alien race in Marvel.