I can't say anything ever freaked me out, but I'm not interested in horror or serial killer type thrillers, so that cuts down on the prospect. A friend who was in love with Dean Koontz's books raved about Watchers and I read it. I enjoyed the enhanced golden retriever, but I regarded the predations of the "monster sibling" and the "disturbed serial killer hitman" as gratuitous.
Probably the "scariest" books I've read were a series in the early 80s by Barbara Hambly. I think it was later labeled Darwath.
Bookmarks