I'll second Splinter Cell: Conviction. Playing it recently has been awesome fun, especially since Danielle and I can both play using the co-op storyline and the various co-op missions. Single player is equally fun.

Mass Effect games will always be the 'big draw card' for me when it comes to the 360, of course these are now multiplatform so if you have a PS3 it's up to you which way you go. I'll stick to 360 because of my saves and due to the entire trilogy being available (ME1 is not on PS3).

Halo games are good, too. But yeah, for the most part it's all about Splinter Cell: Conviction (especially if you have a mate to play the co-op stuff with) and Mass Effect games.