TF2. Team Fortress was almost my first foray into online gaming. (Technically it was Duke3d and then quake, but the former required far too much effort to find and organize games and the latter was brutal for an unskilled noob). Portal was great, but once you beat the game there really isn't anything left to do that interests me. Sure, I COULD try to get the achievements and challenges for efficiency, but that ruins the joy of the game for me.
And to hear people griping about class balance in TF2 is strange. Classes are inheritly imbalanced, because they fulfill different roles and functions. In fact, I would say that classes are more balanced in TF2 than they ever were in any of the previous games because all the classes carry incredible combat potential - some are just easier to use than others.