Quake 2 reallly was the first game that made me start building computers to play video games, so I would go with Quake 2.
EDIT: Hmm... I might have to change my answer to Counter-Strike. From Beta 6 to 1.0 I was totally addicted to that game. No other game since then has been able to top it in terms of sheer playability.
EDIT: Hmm... I might have to change my answer to Counter-Strike. From Beta 6 to 1.0 I was totally addicted to that game. No other game since then has been able to top it in terms of sheer playability.