I would say Defence of The Ancients (DOTA). The most addictive game for me so far. Partly because all my brothers are playing it and i love to play with my brothers.
StarCraft is a very good, and popular PC game, considering it's really really old. It's also addictive.
I agree with DotA & FF7, but dota isn't technically a standalone game, it's a part of Wc3 TFT. DotA & other great custom maps on there make Wc3 one of the best games of all time. The audio on FF7 PC is really weird compared to the PS version, making it kind of bad, if they remastered the audio maybe..