If you're into the more traditional online multi-player, Modern Warfare 2. L4D2 is quite amazing if you're into co-op and have actual friends to play with, but it's style is not for everyone. It's also kind of a drag if you don't know anyone who plays it. I'd also hate to play it if you're only using the 360, because Versus mode would be terrible w/o mouse control. (as infected, mainly)

I'm still playing L4D almost a year later, with literally hundreds of hours logged (and the Zombie Genocidest achivement unlocked...woo, 53,000+ killed). Modern Warfare does nothing more than take up hdd space at this point, despite being an excellent game. It just didn't hold my long-term interest.