I love horror but apparently I don't vibe with a lot of recommendations I find online.
I'd seen so much hype about Event Horizon and I absolutely hated it and didn't find it scary. I just watched Late Night With the Devil and whole it was definitely enjoyable, it wasn't the least but frightening. I also just watched Let the Right One In and really didn't like it. It also was not a horror movie in my opinion.
I will say one of my favorites is Sinister or the first Conjuring. Sinister for the stomach twisting dread and suspence throughout and Conjuring for the same.
While a great movie, I wouldn't necessarily call it a horror. The Thing is a suspense movie based on mistrust and paranoia, but I wouldn't call it scary.