This last position could have been occupied by a number of characters; Lady Deathstrike, Gambit, Juggernaut, Nightcrawler, Havok, Beast, Azazel, Darwin and Sabretooth, to name but a few, but ultimately it goes to the man who defies the odds with his powers time and time again: Wolverine AKA Logan. Played by Hugh Jackman, the character has appeared in some capacity in all seven X-Men movies to date. Possessing the power of healing, as well as a skeleton - including retractable claws - that is coated in the indestructible metal adamantium, he is able to withstand most forms of attack fairly easily (though not without experiencing pain) and can regenerate from the most severe of wounds. He is obviously weak against Magneto, thanks to his metal-coated bones, but he is generally capable of standing up to almost anybody else in a physical tussle (and even if he can't, he's basically immortal once he gets over his injuries) - and this is in spite of lacking any truly superhuman physical attributes. He can cut through almost anything with his claws and that makes him a potential danger to pretty much anyone.
