5) Doom
Like many others, this was the title that exposed me to PC gaming and broadened my horizons. I'm a member of the "Class of Doom". A landmark title in every sense of the word.
4) Unreal Tournament '99
The best multiplayer shooter of the 90s. What more is there to say?
3) TIE Fighter
The only reason this game isn't higher on the list is because it's a very short list. We didn't get another good Star Wars game until Knights of the Old Republic, and that's one hell of a gap. Yes, I know about the Dark Forces/Jedi Knight games. No, they weren't as good as you remember them being.
2) StarCraft
This game is in the running for the game I have sunk the most combined total life-hours into (although WarCraft III may have eclipsed it). Regardless, I don't need to explain it any further.
1) Unreal
I have a spot very very close to my heart for the original Unreal. It's one of the few games that I could accurately describe as "magical" to experience (the only others probably being Prince of Persia: Sands of Time and Half-Life 2). A large portion of this ranking is rosy-colored-glasses, but I really don't give a damn. I'm ok with nostalgia tilting the rating. Another thing I'd like to note is that Unreal was properly long - there was a lot of meat there, unlike today's 8-10 hour shooters. Epic earned themselves an almost unlimited amount of goodwill & trust from this title, which has been helpful lately in overlooking UT3. Winner.