Michael Myers creeped me out a ton as a kid, still does a little bit. I think it's mostly the atmosphere that is created in the Halloween films,
but there's also something about that pale, blank face.
Also the creature from the movie "Funhouse". That movie made me feel so uneasy as a kid. Being trapped inside a carnival attraction with that weird looking thing on the loose would've been pretty scary. It has a nice little Psycho homage at the beginning. I really enjoy that movie, if only for nostalgic reasons.