Amazon's deal of the day is Battlefield Bad Co 2 Ultimate Edition for $39.99 (both Xbox 360 and PS3).
BF:BC2 is by far my favorite online multiplayer shooter. The key is that it rewards team work. Some of my most fun nights on XBL are when I'm in a full party of friends, and we dominate everyone we come across in BF:BC2. It is class based, so if you aren't an amazing FPS player (like me), you can still make a real contribution playing as a medic or engineer. Driving vehicles is great too.