Counter-Strike, without a doubt. If you include all the HL mods (and HL itself), I have probably tripled all other games I played.
IMO, Counter-Strike is the greatest game ever created. If you find a real server to play on, it's the best experience gaming has ever offered. Nothing is like logging on to the same server for two years and seeing the same people, chatting with them, exchanging ideas, and then playing it all out, night after night after night.
Brilliant. Online gamers will never witness that kind of experience again. It's sad how the birth of something can also be its pinnacle.