1. Homeworld (by a long shot)
2. Unreal Tournament (first FPS, still a great game today, pity about UT3)
3. The Orange Box/Garry's Mod (just wow. cannot WAIT for episode 3)
4. Fallout 3 (THE best single player RPG i've played.)
5. World of Warcraft: Burning Crusade (blew me away upon first playing it)