First pc game i ever played was Quake 2, way back in 1998(yeah i'm an old guy!) and i've never stopped since..Once in a while i like to visit the pre-2000 year's gaming walkthrough videos on YouTube, brings back memories.
Many of my young friends here may not have played or heard about the following games but these were the best ones in pre 2000 era(i play FPS games only) which i thoroughly enjoyed playing: Doom, Half Life, Duke Nukem(old one), Quake 1&2, Unreal Tournament, Return to the castle Wolfenstein, Delta Force, System Shock, Clive Barker's Undying, Red Faction, Soldier of Fortune, Serious Sam and many more..
Btw sorry for the extremely long list, the gaming fanatic in me just wouldn't listen!