1. Sid meier's Alpha centauri - Seriously like .000001 cents an hour to play for me. I put a solid year in that game. "Just one more turn" I'd tell myself and it would be 8 hours later and 4am.
2. Planescape: Torment - easily best RPG of all time.
3. Half Life - the king of FPS awesome story.
Honorable mentions - Starcraft, Diablo II, Morrowind, Age of Wonders II, AOE II, HOMM 3, Baldurs Gate II, Disciples II, farcry...and many more.
Maybe cause I'm older, no longer in school - but I never have put the time into games since like those top 3 listed.