If I thought about it, this list would be different. But just popping it out of my head it would be:
1. Doom
2. Half Life
3. Freespace 2
4. Wing Commander 2 or 3 (I know thats 2, cant decide).
5. Mechwarrior 2: Mercenaries
I thought this up in 2 minutes. If I spent anymore time I would have a brain meltdown. Diablo would be up there with number 2 or 3. Warcraft 2 as well. Oh and probably Baldurs Gate. Uh and system shock 2. That conan game, and oregon trail... Team Fortres........ see, these lists stress me out lol.
But DooM is #1 for sure. Ill never forget going into sears and playing doom on one of the display computers. The grotesquely overweight manager would come out and say things like "This aint no arcade boy, get out of here". To which I would reply "Why, grotesquely overweight manager guy, why on earth would you have a computer setting out in the middle of the isle with doom loaded on it if not to attract the attention of curios customers seeking to test said program?" To which he would get angry and I would walk away, only to return and haunt him a couple days later.
I may have left the grotesquely overweight bit out, its been awhile.