Best online multiplayer First person shooter for PC?

Which game is it right now in your opinion and will continue to be for some time?

Team Fortress 2 or maybe even Counter Strike Source or Day of Defeat Source, those will probably never die.

Bad Company 2 is pretty damn good but that will probably die once BF3 comes out. I think people still play Battlefield 2, I wasted a lot of time in college on that.

Team Fortress 2. There is no debate.

Dont kill me but I have been playing BO online Zombies with my mates using mics and stuff and it is really fun. The MP is not bad either, but Team Fortress 2 is better

Team Fortress 2 or Left4Dead2