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...  >_