Korvac
Species: Human/Cyborg
Enemies: The Defenders, The Avengers, Guardians of the Galaxy, Fantastic Four
When it comes to galactic threats, this one is different from the rest. The story goes that on an alternate Earth, Michael Korvac decides to betray humanity to the Badoon alien race. This took place in the year 3007, nearly 1,000 years from now. The race takes Korvac with them but he is caught sleeping on the job one day. As a punishment, they removed his upper body and turned it into a machine. This included alien technology but made him a cyborg effectively. The Grandmaster, as well as the Elder of the Universe, feel that Korvac could be a huge asset.
Therefore, they sent him back in time on Earth-Prime to fight with the Defenders and Dr. Strange. Equipped with future alien tech, he could likely beat the Earthlings fine. Yet he threw the fight, all so he could get close to Grandmaster and scan his cosmic power. Anything Korvac scans can be his, which means he now possessed extreme cosmic power. After killing Badoon leaders, he goes on a quest to conquer the universe. Korvac eventually became so powerful that the only one above him was the “One Above All.” This is literally just the writers and creators of Marvel.