The Gooper Blooper boss? The way he dances and claps his hands to the music (which was awesome) made me feel like he was just so PUMPED to kick my ass. Luckily I accidentally found out that the sponge was his weakness (used it when I was about to be defeated got 3 mushroom roll and completely turned the match around) and mopped the floor with him. This battle alone just may have saved this game for me ;)
"What reason can anyone give me to not prefer the annihilation of all mankind to a scratch on my finger?"