I've heard so many times that Santa Claus is the most powerful mutant. But when did he appear in the comics, and what kind of powers does he have?
It seems he first appeared in Strange Tales #34 in 1955 (although I can't find a scan of it), and has appeared a number of times since. (There are also alternate versions of him in other continuities/universes.)
He first appeared in the X-Men comics, and was first called "the most powerful mutant ever registered", when Cerebro picked up on him at Christmas time in Marvel's Holiday Special Vol 1 1991:
He has several powers, including longevity, the ability to fit himself and others through any chimney, fire resistance, levitation, altering his own or other beings' appearance, changing the size of other objects, teleportation, snow generation, the ability to eavesdrop on anyone and super strength (possibly just on Christmas Eve). He possesses or controls several magical artifacts and beings, like his sack which is a portal to another dimension, magical flying reindeer, and elves.