I'd say DooM, considering I've been playing it, oh, you know, my entire life.
With simple, fun, and addictive gameplay, endearingly dated visuals, sourceports that add more extra features than you can shake a stick at, essentially unlimited modding potential, and more custom content than you can shake two sticks at, it's easy to see why so many people are still playing a nearly fifteen year old game.
Not to mention DooM II was the first game ever played, and I stand by the idea that the second's singleplayer and the first's multiplayer combined make the greatest game of all time.