For me, oddly, it's The Ring. The shitty american Ring that everyone agrees is really not very good and the ridiculous color filter slapped over the whole thing just makes it dreary and washed out and bad.
I know why this is, too. It's because I watched it in middle school before I had any experience with horror movies. The Ring was the first actual horror movie I ever watched and I didn't watch another until after college. As a result, I can't watch The Ring without being transported back to being a scared middle schooler. If I'd first watched it later, after I had a handle on the kinds of scares to expect from a horror movie, it wouldn't have affected me the same. Objectively, it's not that scary and is mostly just kind of bad, but fuck me if Samara climbing out of a TV isn't an image that scares me to this day, every time I think about it!