Not online but I highly recommend the follow xbox titles:
Star Wars Knights Of The Old Republic
Star Wars Knights Of The Old Republic 2 (not as good as the 1st one but worth playing)
Jade Empire (from makers of KotOR 1, and the upcoming 360 game Mass Effect)
Amped 2 (a million times better than the attrocious Amped 3 *spits*)
I think they're the games I enjoyed playing most when I had a xbox. PGR2 as well but we have 3 now so no point.
There's GTA:VC and SA of course (a good 80% of my xbox ownershp was spent playing these), but Saint's Row is out soon so I'm sure people will get their crime fix there now, I know I will!
Never really go into the Halo games (sorry ) but I did have them both. I just wasnt feeling the setting or the story, although I cant fault the gameplay.