Speaking of fun movies to watch while on mushrooms, I once watched Eraserhead and Robocop back to back. Both are basically comedies, but that night, Eraserhead was not.
It's already been mentioned, but The Haunting (the original) is an absolute masterpiece in creating tensions and shock from things that don't actually happen. It's all about the atmosphere, the acting and, slightly unusually, a really creepy use of sets and design that just plays with the ordinary being slightly odd.
Well, If you want plain old sillyness, I have to suggest Troma films, they are really WTF you should be stoned while you watch films. But for classic good ol´splatter I second Braindead, Bad Tast, Evil dead and all that.
And I´d like to add Dead Snow to the Zombie films.
I would say Pan´s Labyrinth but that more of a fantasy really.
If you want a primer on good horror I'd recommend finding A History of Horror with Mark Gatiss online somewhere - may still be on iPlayer. He takes you through horror from the early 30's to the late 70's and picks out his personal favourites. There are a lot of films mentioned, plus it's a very entertaining series.