I think I'll have to agree with most people here that the Stained Glass Demon is my favorite boss. I fondly remember the days when I had difficulty beating that boss and yet nowadays I can defeat him with incredible ease in under two minutes without barely losing any HP. Other contenders are the Demonettes, which also used to be incredibly hard to beat back in the days, especially thanks to that damn tilting platform.
The Pumpkin king. I just had the most fun wasting a giant pumpkin. I love the noises it makes when it gets hit or dies.
All bow down before the master of the vegetable patch, the prize-winning plant who can summon an army of cabbage with a wave of his noble tendril.
He's delicious. He's nutritious. He's Zarok's secret recipe - he's the Pumpkin King!