For me its gotta be between El Chupacabra and The Boogeyman. Both creep me out so much that if I ever think about them during the day, I will actually shut my closet before going to sleep that night sweatdrop
mine would have to be the EL Chupacabra it creeps me out so much and the Boogyman i thing there the one that people us the most in tv shows.
Kelpies are the only mythical creatures that really scared the crap out of me, so I'd have to go with those. It probably didn't help that when I was a kid my mother had a huge art book about mythical creatures of the world, and the Kelpies had the creepiest depictions.
A close second would be Stick-Men, since there's a lot of local folk-lore involving them that goes way back. Just imagining the smiles they're said to have gives me the chills.
Looking through posts, there seems to be a lot of mention of the slender man. I'm afraid to click on any links, but what is he? Is he similar to the stick men or something?