I'm going with Team Fortress 2, its nicely balanced and most of the time completely hilarious and just fun to play. Also I like the fact that each class is like playing a different game, it can be a straight up FPS shooter with the heavy (with added sandvich), a tactical game with the Engineer, a puzzle game with the pyro (though the puzzle is working out where the spy is and then SETTING HIM ABLAZE), or a tense stealth game with the Spy, carefully slipping around the enemy to finally assassinate that medic/heavy combo and then running upstairs to sap the engies stuff, sliding though the air, backstabbing the engie, flawlessly leaping off the parapet to your escape. And then getting hit in the face with piss by a sniper and blown up.
