Either Call of Duty 4, Battlefield 2 or Counter Strike Souce.
Call of Duty 4: Can be good fun, keeps you occupied with the levels, ranks and unlocks etc. Weapon customisation. Single player missions are also pretty epic. It might be just me but I do find myself getting annoy with multiplayer so I don't play it that often.
Battlefield 2: In a sense I'd say it was a more tactical game, if you're playing it with the right people. Weapon unlocks and ranks, the same as Call of Duty 4. Some big servers for some excellent fun. But as I feel with Call of Duty 4, I do feel pretty cheated sometimes on multiplayer but that may just be me. I think BF2 is fun for a while but I don't think it's a game I could commit loads of time to.
Counter Strike Source: Xfire may say different but I still hold the opinion that Counter Strike still has the biggest user base. Loads of servers, and different mods for the game (Hide and Seek, Zombie etc.) It's also a pretty easy game to run, and multiplayer seems to be a place where you can be sociable unlike the other two games where I don't think I've seen people talk that much unless they are saying someone is cheating. Cheaters are pretty ubundant but also dealt with much quicker, because everyone gets spectated at some point, so there's no escaping. I've been playing CSS for a few years now and I'm still not bored of it.
Others will probably say different but we all have our own personal preferences and favourites.