Team Fortress 2
Compared to some other shooters, Team Fortress 2 a bit simple at first glance, with only a handful of maps and modes of play. The nine different classes have a few weapons, which seems like it might not give them that much to do. But the depth of design you discover once you actually start playing is impressive. The wildly different character designs make it easy to know exactly what you're dealing with, which gives the game a more strategic feel than it might otherwise have with such a fast pace. You're going to react differently when you see a soldier coming at you than you will when you see a sniper or a pyro. There's a lot of interesting dynamics, like the common heavy/medic relationship and always being on the watch for a spy in disguise. It's a very addicting, fun game that I'd probably play a lot more if something about spending a lot of time on multiplayer didn't rub me the wrong way.
The game deserves credit for its graphics too. It was a big leap to go for a cartoony style instead of the typical realistic graphics in an M-rated shooter, but it paid off and made the game even more enjoyable. It's really easy to tell who is who, which affects the gameplay greatly, and it's also just cool to look at. There's something much funnier about the content/graphic clash than you see in most games. The amount of personality they injected into the characters of a team-based shooter is noteworthy. I don't know about the community on the consoles, but it's still really popular on PC, so if you don't have The Orange Box, just get it already.
