Anyone seen the trailer for Marvel's Ultimate Alliance? Aside from the heros that are supposed to be a part of the game that you see on the cover, but there's a messload of more that you see in the trailer alone. I'm guessing they're ones that you have to unlock. Xavier, Nightcrawler to name a few.
As far as my favorite goes........
DC - Superman, Batman, Green Lantern
Marvel - Spiderman, Wolverine, Captain America
Love it most when they team up, or even the DC vs Marvel series.