marvel ultimate alliance 2 sucks with solo play but is great in a crowd.
any music game (if you have the gear to share)
I recommend Resident Evil 5 (nothing beats tag teaming melee attacks)
hunted the demon's forge comes out in May (online co-op only)
scott pilgrim vs the world (local co-op only)
any racing game of your preference such as: split second, need for speed, blur, gran turismo
I recommend the sega genesis classic Streets of Rage 2 (it would be great if someone made a current gen version of this fantastic multiplayer arcade brawler)
Dynasty Warriors (pick one they're all the same)
Dynasty Warriors Gundam (wait a minute, that's not a good game, sorry bout that)
Tales of Vesperia (if you import it, damn Namco Bandai)