Marvel movies have some really intriguing alien races. Many of them are extremely mighty and powerful in the comics but the movies show them in an entirely different tone.
A hyper-advanced civilization, the Xandarians were the ones that created the Nova Corps – a race of peace-keeping forces that patrol the universe. The comics show Xandar as an impenetrable fortress and the Nova Corps having some of the greatest warriors the universe has ever seen. In the MCU, the Nova Corps is a mere shadow of what it is in the comics. Xandar is also just like any other alien world. From the Kree Warlord Ronan to the Mad Titan Thanos, the Xandarians have lost too many times already.
The Kronan race was first brought to the limelight during the Planet Hulk storyline. Their home world is a planet called Ria in the Milky Way Galaxy. Known to be powerful warriors, Kronans are found everywhere in the universe from gladiatorial arenas to all kinds of wars and skirmishes in the comics. The comic portrayal of Korg in the MCU makes people think Kronans should not be taken seriously. You cannot be more wrong.
The Klyntar were a species created for war by the Dark God known as Knull. Although they were created to be used as weapons, the race of symbiotes opposed their master and imprisoned him. The Klyntar then became a benevolent race, bonding with other species in order to help them out. The most well-known Klyntar is obviously the anti-hero Venom. Villains like Carnage did give their reputation a bad name. The Klyntar were once a very powerful space-faring race. But in the Venom movies, none of that history is explored. They are reduced to just being a bunch of alien barbarians.
The warlike alien race called the Chitauri were created as a replacement for the Skrulls in the Ultimates Universe. They have the ability to shapeshift, which they use to infiltrate alien worlds and destabilize their social order. After society collapses, the Chitauri come forth and take over the planet. They have far superior strength and speed and a bio-mechanical body that makes them superior to most species. Their technology is also far superior. Despite being a prominent secondary antagonist in The Avengers (2012), the Chitauri in the movies only wield a fraction of the power they do in the comic books.
The Skrulls were one of the oldest species in the Marvel Comic Book Universe. The Celestials once experimented with them and created three Skrull variant species – the Eternals, the Deviant, and the Prime. The Deviant Skrulls exterminated the Primes and Eternals. The Skrull Empire started expanding, conquering thousands of worlds until they eventually came to Earth. The Skrulls are a mighty race that are not to be trifled with. In the movies, they have bene reduced to nothing but refugees of war. Only mere pockets of the race now survive, scattered across the cosmos.
A mighty race once revered as Gods on earth, the Asgardians are the guardians of the Nine Realms. Asgard is lightyears ahead in terms of technology and magic. An Asgardian is far superior to humans in terms of physical and mental attributes. The Asgardian Empire is formidable in the comics. Even events like Ragnarok were not enough to take it down fully in the comic books. The movies literally ended Asgard as an empire and turned the surviving Asgardians into refugees who settled on Earth because they had nowhere else to go.
The Elders Of The Universe
The Elders of the Universe is not one race but a collective of many alien species. They consist of the aliens from various ancient alien races that are almost as old as the universe itself. because they are so ancient, they have developed powers and abilities that make them near God-like. In the comic books, each Elder of the Universe has access to a limitless energy source called the Power Primordial – leftover energy from the Big Bang. The MCU doesn’t even care to explore that part of their history. Characters like the Collector and the Grandmaster got a severe power downgrade. A race of Gods is turned into a bunch of eccentric fools in the movies.