Ok, let's take a stroll down memory lane. Give us the game that floored you, made you give up sleep for days or weeks at a time, or otherwise helped you forget your troubles and totally immersed you in it's world.
My vote goes to....
X-COM: UFO Defense (1994)
What a game! Defending the earth from aliens, what could be more rewarding? It had it all: Strategic depth, a strong tactical aspect, good graphics, very effective sounds, characters that you cared about and cried when they died. You could even name your characters(I named all mine after porn stars).
One of the few games I ever played all the way through to the end. Close runners up would be 1991's Civilization and the original Red Baron.