Writer's Block: Gamer's Choice
I have to say - Wolfenstein 3D.
My dad used to play this game all the time (a little bit more educational than watching lame Leslie Nilson movies, don't you think?) and I've caught the bug too.
But I can't choose only one, so here's a list of my favs:
Mortal Kombat 3
American McGee's Alice (best emo game ever ^^)
Unreal Tournament
And... DOOM, of course! (I used the nickname "IDDQD" a lot, hee...)