For the 360, I'd suggest Halo 3 and Halo Wars (comes out March third), Street Fighter IV, Left 4 Dead, and Overlord. I haven't had much time to try out newer games, but those last three are the favourites floating around school.
Oh, and apparently there's a Halo side-story game coming out in fall '09, where you get to play as an ODST. I seriously need to renew my Live subscription before then.. (yes, I'm a Halo freak.)