Oh, I thought we were limited to PC games here.
Shoot, if we're going all the way back, I'd have to say the first great game I played was probably M*U*L*E*. I keep a Commodore 64 emulator around just to play that game.
As long as we're looking back, I have to give props to Combat for Atari 2600 (two tanks moving around and shooting at each other), and my first favorite game on ColecoVision--Shark, or something like that. You pointed a light gun at the TV screen and tried to shoot a big white square that was bouncing around like a low-quality screen saver. It wasn't much, but the only alternative at the time was Pong!