I wasn't sure, immediately, if I should answer this, because the truth is, I've probably only seen a handful of alleged "Horror" movies. Then I realized that any movie that frightens the ever-loving **&%$# out of you, would qualify.
Not referring to monsters, "walking dead" kind of characters or lots of axe murders and blood....I won't even watch them. Movies like "Silence of the Lambs," "Sleeping with the Enemy," "Cape Fear,"......that are realistic and involve ordinary human beings, who are simply "Horrifying," IMO, make the best scary movie. I'd have to say, "Suspense, and the element of surprise and shock," with extremely evil individuals.
When Julia Robert's insane husband finally tracked her down, and we all knew it was only a matter of moments, that he would pounce on her, I can still remember my heart pounding out of my body. I'd say that's a pretty good horror movie!