The original Doom or Doom 2 on nightmare difficulty.
Am I weird in that I always play Doom, Doom2, Quake, Quake 2, and Duke on the highest difficulties?
Come to think of it- I never did try Doom or Doom2 very much on nightmare skill. I did beat all of Duke3d on *censored* I'm good (skill 4), which is pretty much the same (except doom's difficulties change damage and ammo, whereas Duke3d's difficulties change the number and type of enemies. Both Nightmare and "*censored* I'm good" respawn enemies though (with a crucial difference, if you've blown up the body, no enemy respawns... maybe that's why I had an easier time of it, I made sure to blow up all the corpses).
Doom, Doom2, TNT, Plutonia...
Or
Dear gawd
Eternal Doom...
yep, that's the one right there. Eternal Doom on Nightmare skill. I don't even think it's possible to beat half the levels on that skill without fancy choreography and tool assistance.