As the title states - discuss.
For me:
* Last Battle, Genesis - I dunno how, but I somehow beat this *once*, and never could do it again. Damn tough game. Anyone else here beat this one?
* Ninja Gaiden, NES - I HATE the fact that you have to go all the way back to beginning of the level if you die at the last boss. Again, I beat it once, and never had the strength to even try again after that. Sequels were tough, but no where near the first one.
* Castlevania, NES - Grim Reaper was the one that got me for the longest time. I actually recently beat it again, but had to resort to using save states on a emulator. Man, I can't believe I once beat this without cheating.
* Valis III, PCE - I only recently beat this (within the past few years). Everyone here seems to know about the water level...
* Ghost 'n Goblins - Having to go back to level 1 after continuing 5000 times to beat the last boss for the first trip... Ugh.
* Blaster Master, NES - Another super tough one IIRC. I remember spending hours on this one to get anywhere near the end. Did it once, and never tried to repeat.
Hmmm... I'm noticing they're mostly NES games. I guess that was the era when things were most challenging. And I'm not mentioning any RPG games, as they're mostly just about putting in more time, and not about having to perform amazing feats of reflexes.