Doom ! The first game on which I have spent so many hours, and learned IT in optimizing my config through Autoexec.bat and Config.sys. I succeeded to run the game on a 486 DX2 66 with 4 Mb RAM with loaded drivers ! Lot of fun. I turned 50 recently and am IT engineer now... Though I studyed litterature.
to be honest, when I had Final doom for the psone (first doom I played) I was never scared of the cyberdemon, however I was frightened by the Revenants, Barons, cacodemons, and the pain elementals, because of their disturbing noises and eerie music.