Unreal Tournament & UT2k4 (FUN - Assault mode)
Half Life (FUN, including mods, DoD, CS...)
Starcraft/Brood War (I've played this more than anything, thousands of hours prob)
Command & Conquer (the whole series is the bomb, i haven't played generals though, tiberian universe my personnal fav, but the attack dogs in RA(i think it was RA) were so much fun)
Homeworld/Cataclysm (Great story, gfx, controls were fluid)
Diablo 1 & 2 & LoD (fun stuff)
Warcraft II (fun stuff)
X-COM (we played this at the school library, fun!)
System Shock 2 (Monkey with brains sticking out still freaks me out!)
So many other games..
Today's games don't hold a candle to the great titles of yesteryear, the "young" (cough, i'm 23.. old i guess) generation don't know the greats that came before the clone cr4p that comes out today by EA and M$.
PS. Not considering my old commodore a PC, those had some great games, then my parents got me a nes...