Right now it's Call of Duty 4...and it will be for a long, long time. It is the best online shooter on XBL hands down. The leveling and perk systems keep giving you a reason to "play just one more". 10 out of 10. This game will be $37.00 at Target Dec. 26.
Rainbow Six: Vegas - The community has dwindled a bit since CoD4 was released. It's still a solid online shooter. Great cover system.
Halo 3 - I encourage most people to ignore the haters and try it out. Halo is all about crazy physics and unrealistic combat. The "beat down" is the most addictive way of killing someone EVER.
A warning...
Team Fortress 2 - Is broken at this point. Don't waste your time. It can be a fun game with a bunch of friends, but the lag kills this game every other match.
Edit: Changed Target sales date.