Desert Combat - a mod for Battlefield 1942. Best game balance I've seen. Most enjoyable on-line experience for me, partly due to the players just having fun in-game, and partly because it was my first online team game. That it was new to all of us must have helped with the fun-factor, but it just worked. The balance was spot on, the visuals (of the day) were spot on.... everything about it was perfect for the time and place.
Nothing has come close to that level of enjoyment for me.
Single player - Oblivion, Blade Runner and Civ4