This creepy fuck popped into my head. I remember renting DOA 2 with a friend of mine for a sleepover at his house. MF'er lived deep in the woods and it was pitch black at night. This scary fuck comes on the screen raving about the world being it's delusion and all of a sudden you're fighting.
Very intense for the boob jiggling horny fighting game.
Vagullion from Ys I is pretty bad, the timing for hitting the boss is strict and the bats move in such random patterns that a lot of it comes down to pure luck.
Any boss that has multiple forms and you have to start all over after dying. The first boss from Sonic 2 on the Game Gear, incredibly unfair.