Far Cry 3 - the co-op missions are awesome fun. Danielle and I played them all and the ended up replaying them for weeks, it was just good ol' splitscreen fun. You'll get two other people thrown in with you but that keeps things interesting. I don't know about 360, but on PSN there wasn't any voice chat function which worked for us because we don't like annoying people talking to us while we game. But yeah, great fun.

Beyond that... Rayman, as mentioned by Shauna, was epic fun. I know there is a new Gauntlet game out and I'm unsure if it's on XBL, but if it is, I hear it's pretty good.

L4D, L4D2, Halo 3, Halo: Reach were all fun enough to play. Can't remember anything else off the top of my head.