Freddy vs Jason was a mixture of two of the best movies I've ever seen and it was going to be so good but I feel like they ruined the opportunity. If you've seen it you'll probably know what I'm talking about.
The scariest movie I have ever seen that actually bothered me was "Sinister" that movie is screwed up. Pointless torture. But it was not too bad. If you really want to scare yourself, do not watch a movie, play with an ouija board or go observe an exorcism haha. Proceed with caution with those.