Bowser of the world 8-4 and the squirrel boss in super mario bros. and donkey kong country.
Bowser took me a lot of time to defeat, because he is always launching things at my face, but with the axe i could defeat him.
And the squirrel boss, it took me a loooooooooooooooooooot of save states to beat him, he is really annoying to defeat, but at least with donkey kong i could defeat him no problem, he can be really hard to beat if you just jump at him, but when he is defeated, that is satisfying.