1. Doom II: Because it's perfected Doom I with double shotgun, much crazier monster madness and map design. I spent countless hours on this game. It's shame that even TODAY there's NO gameplay like it. No, Serious Sam and that other gothic FPS don't count- they're not even close in polish and killing meaty monsters.
2. Starcraft 1: There are games I spent more hours on it like Diablo II. But SC1 left so much more impression in me- amazing lore, great single player, awesome soundtrack... everything is right.
3. Diablo 1: Again, it just imprinted in my mind more than II. 1 was just.. mysterious... that Tristram guitar sound, small-town NPCs, and Butcher.... DII may have a superior gameplay, but this game is just magical.
4. Duke Nukem 3D : Just an awesome game. Lots of people just play this game at face value and only first act. But act II & III are just incredible. Map design is pure genius and some level are actually quite dark and eerier than Doom.