Hmm, these lists are always so damn hard to do.
Anyway, my list would probably look something like this:
1. Civilization 2 - No other game has "stolen" so much of my time
2. Fallout - Best CRPG so far.
3. Pirates! - Two Sid Meier games in my top five, this game also occupied a lot of my time, first on my computer and later on the illfated Amiga CD32.
4. Close Combat - The entire series actually, since it so hard to separate the games.
5. Doom - This is still the only FPS game I've really, really liked.
6. SimCity - I like building stuff.
7. Planescape: Torment - While it sometimes played like an interactive novel, it was a very good one.
8. StarCraft - Best RTS game I've played
9. Wasteland - Somehow, the list wouldn't seem complete without it-
10. Arcanum - This could easily have been a top five game, if not for the combat and other gripes I have with the game.