I like Halo 3, lots of people seem to hate it for some unknown reason but overall it's a solid FPS. It has good online play, 7 downloadable maps, and is great for local play with friends. If you don't like listening to the idiots on Live then just mute voice communications or don't play on Live (I think it's best with friends, either splitscreen or System Link).
I think GTA4 is fun, though I haven't played it much (a bit on 360 and some on PC). It's fun trying to get away from a high wanted level but I never got too interested in the story, more fun to just drive around and have fun.
I too am looking for some new games, just got a new 360 after giving up on my hacked, repaired, free one and now I want some more games...I have Rock Band/Guitar Hero and Halo 3, and in a dorm room there isn't any space for GH/RB so I'm stuck with Halo, which, like I said, gets boring unless you have people to play it with.