My favorite boss from one of my favorite games, Metroid Fusion.
It wasn't the creepiest boss in the game in my opinion but still... he's awesome.
And I wanted to have slime coming out from his "eyes" but I couldn't make it look good so I had to skip it.
I liked the metroid bosses from the older games, they were monsters and some of them looked really creepy. Now, it feels like it's just bounty hunters and a Samus clone.
Nightmare (c) Nintendo.