Worst: Pumpman......wtf this design!
He "pumps" himself up before battle, literally (bu-dum-tish!) lol.
I like them all, but I'm only going by boss battle (not design).
I found once you keep shooting him, Toad Man can't do a lot to you.
I mean, fair enough, his rain flush is unavoidable when he uses it, but yeah, once you got him where you want him, it's just a matter of time (and not stuffing up). I know I'll get some hate for this, but that's just what I think.
And I'm not having a go at animal based RMs either (just look at my avatar, lol). Also, while Sheep Man did sound ridiculous to me when I first heard about MM10, I absolutely love that guy, lol.