I would have to say the Old Chateau. Although I have never seen one myself, the ghosts that float without saying anything around are a creepy concept. The painting with the red eyes is also VERY unsettling (who knew that two red pixels could be distubring
), and the music creates an appropriate feeling of disarray and "wrongness," as if you aren't meant to be there.
The Strange House would be my runner-up, for similar reasons. The rumbling furniture was an excellent touch.
Also, while not overtly "scary," I do find the Hall of Origin to be fairly unnerving, especially with its distorted music. Whereas I actually find the Distortion World to be quite soothing.