3. Mister Sinister
Marvel ComicsMister Sinister is an all-round great character. He's incredibly powerful, has been a key component in the origins and manipulation of several important characters in the X-Men comic books and has links to Apocalypse that would make him a fairly obvious follow-up villain to the ancient blue mutant (he has impersonated Apocalypse, made pacts with Apocalypse, joined forces against common foes with Apocalypse, gone directly up against Apocalypse - such as the time he created Cable specifically to destroy him - and, most importantly, he was essentially created by Apocalypse). In fact, he may well appear as a sort of sidekick to the big man in X-Men: Apocalypse itself. Sinister possesses superhuman strength and durability, a decent level of telepathy and telekinesis, a regenerative healing factor and the ability to manipulate matter at a molecular level. He is also a very formidable hand-to-hand combatant and has fought comfortably against Deadpool whilst one-armed with a baby in his other arm. His arrogance is such that it is almost likeable and his status as a genetically altered human (a mutate - which is a completely different thing to a mutant, incidentally), rather than being a mutant from birth, makes him somewhat unique amongst X-Men characters.