Tomb Raider (awesome game for it's time)
Carmageddon (splat!)
Fallout (what got me into PC gaming in the first place)
Syndicate Wars (corporate management was never more fun)
X-Com Apocalypse (yes, I actually liked that one)
Defender of the Crown (the game was so simple, yet so addictive)
Lords of Midnight (the ultimative 8 bit game)
Master of Orion 2 (will we ever get anything like that again ?)
Ultima IV (first crpg to truly capture my imagination and getting me lost to the world in a computer game)
Elite (Bell, Braben, you suck. Shake hands, make up, make a new game dammit!)
All of those are old games, some of which had sequels, but to the best of my knowledge more sequels in name than anything else... >_