I think the "Dead hand" from OoT is the most terrifying monster ever x-x I don't know why since I know there are plenty more even scarier-looking monsters out there in other video games, but they still don't make chills run down my spine the same way as this hellish thing does... (Or well, there's always "The pale man" from Pan's Labyrinth, but that's a movie and not a video game >->')
I guess it's the whole "Lots of random hands holding you down while some random creepy monster-thing slowly inches towards you ready to bite your neck off and there's not really much you can do about it"-thing that terrifies me so much. Most other monsters are usually all "Surprise!!! Here I am!!! *Nom*" and while that can be scary too it's more the surprise of the moment than the whole situation that's scary.