Castle Crashers is at the top of the heap. It came out as part of the first Summer of Arcade, and it's been the best-selling XBLA game ever since. Remember how we said SoA was console gaming's biggest downloadable game promotion? Even if every other game it pushed was terrible, it would still be true because of this one game.
On paper, Castle Crashers doesn't seem all that special, especially in 2012. It's a beat-em-up with four-player online or couch co-op, featuring an RPG-style leveling system. That leveling system, persistent through multiple playthroughs, encourages you to play over and over by adding new characters with unique abilities.
Like every game on this list, personality is what sets Castle Crashers apart. In addition to coming up with a number of variations on the game's basic gameplay, CC is chock-full of large, crazy-looking enemies that you might almost feel bad about whacking them into oblivion.