Who is the biggest of the bigs, the baddest of the bads, the one among all others that the X-Men look upon as their biggest foe?
Apocalypse
Argument for: What is there left to say? An evil from the earliest tenats of history, once described by Charles Xavier as the "Living Embodiment of Evil", destroyed worlds, realities, and has generally been a world class thorn in the side of every good guy since forever.
Argument against: Sort of dead at the moment, and has been for a while. Subjected to the Worf Effect heavily before his demise.
Mister Sinister
Argument for: Tortures, experiments, and ruins mutant lives as a job. A psychopathic mastermind of terror and carnage, and always a threat no matter when he pops up. Tends to stay dead even less than most.
Argument against: Less an "X-Men" villain, and more a terror of the Summers and Grey family lines.
William Stryker
Argument for: Debateably the most hate-filled individual, a preacher using his pulpit to torment, torture, and lead assaults on innocent mutants.
Argument against: Not really dangerous on his own merits. Also came down with a nasty case of dead.
Nimrod/Master Mold/Bastion
Argument for: For the longest time was the apex predator of mutants. The lengths to which it took mutants to kill a single Nimrod is frightening. Bastion encompassed the worst of both.
Argument against: Bastion ate the nerf bat at some point, and has been progressively less powerful in each appearance since his debut.
Magneto
Argument for: the most iconic X bad guy, has done more damage than just about anyone else
Argument against: Not really been a bad guy (at worst morally gray) for about a decade now.
So what are your thoughts. Someone here? Someone not on the list?