I enjoy using Iron Man, cuz he's got everything...flight, might, ranged, and great boosts (with probably the most effective Radial Xtreme)
Iceman is arguably the strongest character in the game, so no question 'bout that.
Nightcrawler's a teleporter, and every team needs one. His Xtreme Master of Chaos is definitely worth the Xtreme token. Ultimate boss killer, like MK's Lunar Eclipse.
Magneto's just a fav of mine, cuz I just love the power in which he pulls objects and enemies toward him, right into the waiting cyclone of Nightcrawler's swords.
Sometimes, I alternate in Gambit, but I rarely do, since I enjoy using all four of my characters.