Most modern horror movies that throw guts and blood at me make me more angry than scared. It enrages me to see suffering trivialized (talk to my therapist ;) ). I usually walk away or turn them off (like the Ring was a total bore). If they involve creatures or animals I am usually rooting for them and not interested in the people surviving.
And like others have said, I can't suspend my disbelief long enough to get into the movie if it has stupid people doing stupid things (like all I could think during Blair Witch was "YOU IDIOTS!")
Scariest movie ever for me was Psycho (or anything Hitchcock). Or Nosferatu. Or the orginal Frankenstein or Dracula. OR the true story of Count Vlad or Elizabeth Bathery. :)
That reminds me of a joke I heard about Vlad, it goes something like this-
So this bar goes into a guy...