1. Pokemon Blue/Gold, both great games, clocked 80 hours into each as a kid.
2. Halo:CE, my first FPS, great level design and epic story.
3. Fallout 3. Addictive, massive map, but a crap ending. Bought it again (the GOTY) just for the DLC.
Also Mass Effect, Assassin's Creed 2, CoD 4, Batman: Arkham Asylum and GTA IV have been the other single-player hightlights of this generation.