Viewtiful Joe on Adult for me. I know many people have finished the game on Adult and the two further difficulties, but Fire Leo is frustration on its own level.
It's not that the boss itself is hard (I mean, he is, but that's not the only problem), usually when you have a hard boss in VJ, you keep trying, restarting from the beginning of the match over and over till you finally manage to beat him.
With Fire Leo, it's hideously annoying because a game over means you restart the whole stage, which just so happens to be a boss marathon fight against all the previous bosses in harder form (without Alastor, though) before you actually get to Leo. I know how I'm supposed to beat him, I know how to dodge the attacks and make him spin and them lay it on him and then back away and I know he goes even harder when you finally get to the very last bit of his last life bar. I've just never managed to do all that in one go without slipping up.
I remember starting the game on Kids, thinking it was "normal", while Adult was "hard", played for five minutes, restarted on Adult, played all the way to Fire Leo and after a week of trying, gave up. I later restarted on Kids and played through the game without a single continue. The gap between Kids and Adult was huge for me, and while I hope I will one day finish it on Adult, I don't think I'll be getting past Fire Leo any time soon.
The rest of VJ is hard but plays fair. When you die, you feel it's your own fault. But with Fire Leo, while I know I'm just not good enough, actually getting good against him is unnecessarily bothersome since you spend half of the tries fighting the other bosses and not him.
As for anyone wondering about Ninja Gaiden 3 should check out the
SDA run on it (non-emulated).
Jam it back in, in the dark.