Starcraft BroodWar. That game is old, but hell, you can't argue that it was the best game of it's time, and it's still fun even today.
I also used to totally kick ass at Unreal Tournament (original) assault (I was one of the best out there, really), but 2004 really messed up assault, so I just playing some onslaught, and gave up on that after a while.