For me it has to be Quake. I was a bit too late for DOOM, but before then I was playing as a child on the Sega Megadrive (Sega Genesis for my US pals) and going from the Megadrive graphics and gameplay to Quake...

I think that was the first time I was absolutely addicted to a game. Like, I was pretending to delete the game and hiding it using Explorer's hide folder mode so I could secretly sneak some Quake in here and there.

Absolutely love that game.