Stanley Kubrick's "The Shining" is quality; even if parts did differ from the book a bit. The imagery in that movie is great.
But the movie that scared me the most is one that I'm sure only 1% of the people on this site have ever heard of. It's a 1978 horror movie I first saw as a kid called "The Manitou". Here's the IMDB description:
"A psychic's girlfriend finds out that a lump on her back is a growing reincarnation of a 400 year-old demonic Native American spirit."
Sound cheesy? Yeah, maybe it is story-wise. But once that thing emerges from her back, it's pretty damn scary. First, a Native American Shaman is brought to the hospital where the woman is to try and contain the spirit before it's out. Needless to say, that doesn't work too well. Second, because of all the X-ray radiation done by the doctors, the reincarnated spirit, called Misquamacas, comes out physically deformed... and he's real pissed about that.
It's not the best horror movie ever made, but Black Magic freaks me out, and it's directed well regarding the scare factor. In particular, the part where Misquamacas re-animates a dead body (mostly the sound it makes) still creeps me out to this day.
Seriously, even without Medicine Man magic, Misquamacas would be terrifying: