4. Apocalypse
Born in Egypt, En Sabah Nur (which means The First One) is made up of organic and powerful celestial technology. During his travels, he stumbles across advanced alien technology and uses it to enhance and transform himself into an even more powerful being. As one of the first mutants ever created, Apocalypse believes that only the strong should survive. Through out time he battles with the X-Men and the Avengers in his quest for ultimate power. During his thirst for power, he recruits four mutants as they become the Four Horseman of the Apocalypse, one of which is the famous Angel. In the X-Men Evolution series, it takes all the X-Men as well as Magneto and his band of mutants in order to defeat the all mighty Apocalypse. He is by far the X-Men's toughest adversary in history. His powers include immortality, super human strength, invulnerability, teleportation, flight, size alteration ability, super stamina, and is an absolute genius among other things.
His Weakness? When at full strength, we've yet to see him be defeated but with that being said, he has to occasionally regenerate himself in order to sustain his massive powers and during his awakening after regenerating is when he'd be most vulnerable to being attacked. Other than that, he's practically indestructible.