Out of every FPS game ever made, I have to go with Doom.
IMHO, Doom set the standards for first-person shooters before the Halo, Half-Life, Call of Duty, basically any FPS game today. This game was the second FPS game I've ever played, the other being Wolfenstein 3D. I've enjoyed blasting my way from space to the depths of hell with my arsenals such as the Shotgun, Chaingun, Rocket Launcher and most notably, the Chainsaw, running at 10-15 frames per second back in the mid-90s, if I remember correctly.
Great memories from this game.