Can we mention atleast a boss that ISN'T from an RPG?
That last dude from Mortal Kombat 1. Remember punching my shitty retro console because of how bloody impossible it was. The thing that made it even more annoying was the fact there was only like four possible moves you could do, excluding the special moves. So what the AI would do is constantly kick you down, over and over again so you couldn't do ANYTHING.
RAGE.
Also, The Blind from Metal Gear Solid: Snake Eater. That was a long, gruelling challenge that requires a lot of thinking through to actually accomplish.