Books

I mean, there are so many kind of horrors.

The most obvious pick would be Stephen King: his best-sellers are so for a good reason: they're good. Maybe consider Bags of bones, or his classics: Carrie, The Plague, It,... I mean is much longer Dark Tower (not really horror, imho) is something I would love to not have read already... so I would be able to read it for the first time one more time ;)

Haunted house story: like King, I consider the Shirley Jackson 'The Haunting of Hill House' a masterpiece. In French, Jean Ray 'Malpertuis' (sorry I've not read any English translation). I think I prefer Jan Ray but being French myself I may be biased, here, and I still love Jackson's novel.

Another classic in his own kind: Edgard Allan Poe. I mean, come on :)

Edit: as suggested already: +1 for Clive Barker short stories.

Since you've be careful enough to put limits on what you're looking for, I won't mention the real 'horrifying' horror writers I can think of, but be aware horror stories do not end to best-sellers nor to classics.

Among the most... intense, there a few writers that come to mind whose names I will never publicly suggest to anyone, even though they're wonderful craftsmen, like really... but they can also be deeply and utterly disturbing which, in our age filled with angry self-appointed vigilante always on the lookout for someone they don't like to lynch, is something I would never want to happen to them. But they're there, alive and writing masterpieces, some of which will undoubtedly become tomorrow's classics... if there is anyone left to read books, that is.