I'm going to have to go with Counter Strike.
The skill gap is like the Pacific Ocean, and it really shows off when you improve your game a lot. It's incredibly challenging playing against other skilled players, and extremely rewarding when you win with good tactics and good communication.
IMO, there is no game out there that reaches CS's height in competitiveness.
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Edited by o0MrCheesy0o: 4/17/2013 3:21:09 AM[url=http://www.quakelive.com/]Quake Live[/url] - Counter Strike is random, campy, and breaks down when played in anything other than 4v4. Quake has more depth, and a larger skill-gap. It's [i]the[/i] competitive first-person shooter - it's not just about aiming and movement, but resource management too. It's a unique, fun combination.