I think horror films should be split into two categories, as there are a number that are more entertaining and rewatchable and some that are just for a one time viewing.
Horror does get the most criticism unfairly at times when it comes to rating from the critics than any other genre I feel.
I think the most scary and dark atmosphere horror film I've watched was The Entity (early 80s) Barbara Hershey, the music played a big part aswell but it was apparently based on true events of a woman who was getting sexually assaulted by a demonic poltergeist, at the end a scientist believes her and helps to try and catch it and it' shown in it's form for the first and only time to be this massive insane energy ball of light that is 40 foot high and the sound effects give it more tension, but it's strength allows it to escape the trap but it's a film that sticks with you.
But the horror films I enjoy the most and rewatch even if they're less scary are - Nightmare on Elm Street, The Lost Boys, Jaws, Dog Soldiers, Scream, The Thing and any of the Stephen King adapted movies even if alot of them were b movies.
The quality of horror films is not as good as it used to because the heavy use of CGI has often ruined alot of the modern ones and made them look less creative but there still has been some good ones in recent times - Bone Tomahawk with Kurt Russell 2015, Cabin in the Woods was decent and different. It Follows was good 2014 and I liked 'Smile' which came out last year. The Cube was a good film late 90's and they made a few more likes it recent times like Circle and Belko Experiment these are moral dilemma horror films similar to the premise of Saw.
I think the more recent ones that have been good have the antagonist using more psychological tactics to break down the protagonists mentally rather than the relying on the old ways of stab and kill.
Even the IT remake was alot like that and it was fairly good, Tim Currie is obviously a legend but Skarsgaard played IT very well.