Hardest games

[Protoclown]

I'm thinking of possibly doing a piece on hardest video games, so if anyone has any extremely difficult games to suggest, I'm all ears. I'm thinking of oldschool teeth-grindingly hard games from back in the day, because even the hardest games these days look like a walk through the park compared to some of those.

So far off the top of my head I've got:

Castlevania
Ghosts & Goblins
Mega Man
Battletoads
Ninja Gaiden

In cases of series, I'm only going to include one game that's representative of the whole. Feel free to suggest others, though I'll be more inclined to use games that I've played or am familiar with...though if I can download a ROM of stuff I'm not familiar with, it might still be included.

I KNOW there are plenty of other games that had me foaming at the mouth in a rage, but this is all I can remember at the moment.