Mike Tyson. I couldn't stand him, since he was my first boss fight >_< I eventually learnt the pattern, but I wasn't used to bosses as hard as that one

(And yes, I know he was the easiest one xD)
Also, I used the S key to skip some animations during the fight. It's handy at the beginning, but after that it resets the battle every time you press it :(
Willy was a pain in the ass too. Bowser and Wart were really easy, but you had to play through them every single time you got killed fighting Willy, and that sucked >_< Specially when, at first, I thought you couldn't hurt him while he was dropping the ball (Since I kept trying to do it from the left side of the platform, and it was out of range)...
Oh, and Kirby's Nightmareland before I learnt how to walljump properly (Instead of keeping Shift pressed, I pressed it just before jumping. So it was a lot harder, but I still managed to do it... Except for the last jump, which is when I learnt how to do it properly, a bit too late >_<).