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.
"Ju-On: The Grudge" from 2002 was probably one of the scariest movies I've seen (if you don't mind reading subtitles). The US remake did not do it justice IMO.
I don't know how well it holds up nowadays since it's so old and I was a lot younger when it came out.
I also remembered it as insanely scary, but when I rewatched it recently it really didn't live up to my imagination. Maybe I had hyped it up too much in my head