I personally find Dolls, and Clowns to be the scariest two, mainly because of the creep factor. A vengeful spirit isn't that scary unless its a poltergeist, more of a nuisance then anything else. A zombie horde (Romero style) is easy, as long as you don't get cramp or a stitch just walk the other way, the other faster kind are admittedly very scary. Mutants and monsters can be very scary, except very few people actually make them scary, and if they appeared in real life there is a very good chance you would come across a human body on a rabbits head, and because of that, they are too funny to be the scariest. A murder is scary because its the only one that could really happen (except the Clowns, but then they would become murders) but there isn't not much left to the imagination. And aliens aren't really scary anymore, they all look like the same basic shape as humans nowadays, and are just a bit rubbish.