I'm a late comer to pc gaming, so my list would probably be a zillion old 8-bit titles...
Some all time favs:
Lords of Midnight/Doomdarks Revenge, the best turnbased strategy/rpg mix ever. Has actually been remade in a great pc version. I'm still beating the crap out of it
Forbidden Forest, the best survival/horror game ever. Resident evil go home. This game still gives me goosebumps when reaching the final stage... :ph34r:
Ultima I-IV, doesn't need a lot of explanation
Fort Apocalypse, outstanding action game. Can anybody spell claustrophobic ? Trying to navigate your chopper above and below ground, dodging flak and missiles. Had that magic mix of being frantic without being unplayable
Seven cities of gold/Heart of Africa, from back when EA releases were looked forward to.
Elite, the first real space exploration/trading game. Originally released on the Acorn BBC computer. The first game that gave me a feeling of total freedom, where do you wan't to try your luck today ?
Boulderdash, I'll be damned if you can't find a handful of cheap clones somewhere out there, I just never did, the idea is so simple...
Lets see, that was a small handful from my wishlist of remakes, only a few hundred more to go
Bah, who cares. Most of these titles are from before half of the board members were born...