If you like shooters I can't see anyone NOT owning, theres just no excuse. -
Rainbow Six:Vegas
Call of Duty 4
Those 2 I'll never trade in.
Gears of War should be gone through at least once. If your first experience with a Bezerker doesn't hook you theres something wrong with you. The multiplayer isn't as great as I'd hoped and theres no real replay value.
Bioshock - Yeah best games I've ever played.
FEAR was ok on the PC and I played it a bit at a friends house on his 360....
not very exciting. Maybe a rental. There are great shooters on the 360, FEAR isn't one of them low price point or not.
Need for Speed Prostreet - I just recently picked this up. I like you am not a big sim racing fan. The NFS series have always gotten my $ going back to NFS 2: SE for the PC. Ahhh Mcleran F1 how I miss you. I haven't got real deep into it yet but what I have played is fun.
Guitar Hero 2,3 and Rock Band - Now some people just are not good at these games but damned if they don't keep playing them. You will suck guaranteed when you star on easy but once you start nailing songs you will be hooked. These are my pick up and play games.
XBL Arcade has the original DOOM! I've had a blast playing this and going through it coop with friends.