Most Creepiest? Damn, that is some elegant speech.
Redeads in Ocarina of Time used to make me cry when I was a child. They were pretty damn frightening.
Mother Brain from Metroid was creepy, a brain with tentacles and an eye, floating in a jar, that's just creepy.
Two Back from Silent Hill Origins scares the crap out of me.
You see, as it walks, it screws itself, literally. It is apparently based on something Shakespeare wrote about sex, something along the lines of "like a beast with two backs". Whatever the reason it does that, it is still creepy.