Rivaling the cosmic scene of both Star Wars and Star Trek, Marvel Comics has been building up its component of extraterrestrial worlds and characters since 1962's Fantastic Four #2, which introduced comic book readers to the shape-changing Skrulls. Slowly but surely, the Skrulls would be joined by two other powerful alien races, which would make up Marvel's trinity of unequaled space empires, the Kree and the Shi'ar. But which of these alien empires poses the greatest threat?
Hailing from the Andromeda Galaxy, Marvel's Skrull race relies on their natural ability to shapeshift into other members of a species in order to overthrow other races. Following their initial defeat at the hands of the Fantastic Four, they began to produce their own Super-Skrulls. Super-Skrulls are members of the species reverse engineered with the abilities of super-powered beings from Earth as well as other areas of the galaxy. However, the Skrull Empire saw their numbers drastically dwindle when the feared cosmic entity Galactus Devourer of Worlds came upon the Skrull homeworld of Tarnax IV, living up to his namesake and consuming the planet. Surviving Skrull Veranke would eventually ascend as ruler of her race and attempted a secret invasion of Earth, which was ultimately thwarted by the might of The Avengers. The Skrulls have been embroiled in a centuries-long war with another of Marvel's big three alien races, the Kree Empire.
Requiring a force of their own to combat the Skrulls, the Kree would begin experimenting on humans on Earth with the hope of unlocking a concealed capacity for superpowers, leading to the creation of the offshoot race known as The Inhumans. Located in the Large Magellanic Cloud, the Kree largely act as a militaristic species akin to the Skrulls, led by the precognitive A.I. The Supreme Intelligence. The Kree's main enforcers are their Accusers, led by villain Ronan the Accuser, who regularly acts upon the destructive orders of the Supreme Intelligence. Conflicting with Ronan and The Supreme Intelligence was Kree commander Mar-Vell (Captain Marvel), who would end up passing his moniker and powers onto human pilot Carol Danvers. Along with their own military force, the Kree are in possession of massive robotic Kree Sentries, capable of emitting energy beams and increasing strength to fight any particular threat if needed.
Though not as ancient as the Kree or Skrulls, the Shi'ar Empire remains Marvel's most menacing alien race to date. Avian-like in nature, the Shi'ar watch over their side of the galaxy from the planet Chandilar. Acting as the empire's defenders are The Imperial Guard, consisting of several super-powered beings from various alien races, led by the majestor Gladiator. Unlike their predecessors, the Shi'ar possess a secret weapon that vastly outclasses any Kree sentry or Super Skrull, in the form of the M'Kraan Crystal. The ancient M'Kraan Crystal acts as a gateway to another dimension between worlds, capable of destroying the universe itself.
The covert skills of the Skrulls may be cunning but they are no match for the Shi'ar Empire and their M'Kraan Crystal. Though the crystal can only be activated once every million years, Gladiator and his Imperial Guard have successfully conquered Ronan the Accuser as well as Earth's superheroes numerous times. The Kree and Skrulls set the groundwork for Marvel's hierarchy of alien empires but the Shi'ar obtained the concept and have been running with it ever since.