None of the books themselves scared me, only certain moments of particular stories, most notably the Sun Dog and Bag of Bones. But for books that are filled with the raw horror element, I would say that the top 5 that most especially fill that criteria, in order of publication, would be 'Salem's Lot, the Shining, Christine, Pet Sematary and It. Stories like the Stand, the Dead Zone, Cujo, the Tommyknockers, Desperation, Duma Key and others have moments of true horror, but there are a few other themes running through those stories that keeps them from being in a "pure" horror genre as the 5 I have chosen are.
I think that the one that has the most horror element based on human terrors with no boosts from the supernatural in the parts of the book that are scary is Rose Madder. Norman's obsessive pursuit of Rose and the lengths of violence he will go to in his mad quest struck me as far more frightening that most of King's supernatural-based stories and for me at least shows a far better example of how a normal human can get carried away and go too far than some of King's other human villains, such as Greg Stillson and Annie Wilkes.