Nightcrawler is clearly the best because of his jump kick and teleport ability, Cyclops has a good projectible mutant power as well as a high jump, Wolverine heals (at the snail's pace of just a fraction of his health every 3 minutes or so) and his claws augment his attacks, but Gambit's mutant power (throwing a charged playing card) isn't useful

Nightcrawler simply is the best (and sometimes the only useful character) to choose because of his extensive, yet deadly teleportation and superior speed