the first Quake. The way I had it set up there wasn't any music, which differed from the other FPS games I had at the time, Duke 3D and Doom (scary in a "hey, quite biting me, you gosh-darn demon, that hurts slightly"). God knows how many times I had to re-do segments involving those bloody shamblers in Nightmare skill. I go around a corner expecting to pop a knight or two, and be greeted by a shambler:
Me: Die! you *censored* knights! hey wait a sec.
Shambler: roar.
Me: Eeek! die you demon spawn!
Shambler: hey.
Me: Run away!
Shambler: hey no fair I can't fit down that hallway
Me: ha that put you in your place. eat Nails!
Shambler: ack you have killed me.
Me: yay!
The shambler,baron of *censored*, and battlelord have got to be some of the most hated enemies (by me) in Quake,Doom, and Duke3d respectively.