The test of a mutant's powers is not who you could beat up, it's how long you last with those powers. Everytime there is a character who is beyond powerful, they die or lose their powers.
Franklin Richards-stripped of his powers when he created another world to save the world's heroes (people, he created another world that was on the exact opposite of the sun at six years old!!!, DEACTIVATED)
Nate Grey/X-Man-became a god-like shaman to watch over the world, basically a force of nature, so that he doesn't run into any of the other characters. (WATCHER, NON-ESSENTIAL)
Kevin MacTaggert/Proteus-can possess anyone, and that includes their powers. Too powerful, had to kill him (DEAD)
Calvin Rankin/Mimic-in regular continuity, he is a wasted hack...but in Exiles, he uses his ability to hold any five super powers at once to it's fullest. He is warped around to different universes and never stays in one place (FLUX BETWEEN DIMENSIONS)
David Haller/Legion-He was crazy, but in "Legion Quest" all his personalities which had different powers became whole. Telepathic, telekinetic, pyrokinetic, and time travel all at the highest order. Basically sent Bishop on a quest to kill him before he killed his father Prof X and changed the world (DEAD)
Jean Grey/Phoenix,Dark Phoenix- Celestial being of destruction, a cosmic force. Had to "kill" herself to save earth (actually the Phoenix Force is about to make a comeback NEW X-MEN #148, ortherwise DEAD)
Legion would normally always have the advantage because he could travel back in time and kill your parents, before you are born, but Mimic could take that plus four other powers. But at the end Proteus wins, because he could possess any and everyone and use their abilities. The others are still, or would be extremely powerful, but the ability to use one's powers' against you and others trumps all.