Nightmare Fuel: The bosses, without a doubt. Special mention goes to the Gate Keeper, a huge ogre who attacks you with his teeth and has an especially menacing theme. It’s telling that the final boss is the least scary of them.
The fact that you have to rip the bosses eyes and noses from their faces doesn't help either. Not even the Savage Pumpkin, which doesn't even have eyes or a nose, is safe from this