What is Classic Horror?

Around Halloween time, a few patrons came to the library wanting to read novels of 'classic horror.' I recommended what I could, but the past few weeks have really left me thinking... what is 'classic horror' ?

Firstly, I have to define 'classic horror' ... is it horror that contains classical elements? Or is it simply horror that is well-known and well-written? If it must contain classical elements, then I get to throw out a bunch of things... namely sexy vampires and werewolves, zombies, and extraterrestrials.... which leaves us with elements like ghosts, satanic (or in some minds violent pagan) rituals, demons, and possessions.
The immediate authors that come to mind are...

Edgar Allen Poe's Tales of Mystery and the Imagination
H. P. Lovecraft's The Dunwich Horror and The Call of Cthulhu 
Matthew Gregory Lewis's The Monk
Arthur Machen's The White People
William Blatty's The Exorcist

... but there are many who are likely to argue that medical horror would have classical elements so that means I need to include...

Robert Louis Stevenson's The Strange Tale of Dr. Jekyll and Mr. Hyde
Mary Shelley's Frankenstein
H.G. Wells's The Island of Dr. Moreau

... but some would argue that these aren't traditional horror. These are truly borderline science-fiction / horror novels. So, I feel obligated to say they are sci-horror but not classic horror.

And what about novels like Bram Stoker's Dracula ? - there's a little too much of a love/drama element keeping it from being truly terrifying, even if it is Vlad the Impaler we are talking about. In order to be horror in the classical style, it needs to be horrific throughout.

And novels like Anne Rice's Interview with the Vampire or Stephen King's The Shining or Cujo.... they aren't written classically. Granted, they have a cult status and if they persevere long enough, I'm sure they will make it into the canon of classic literature.

Splatterpunk novels like Richard Matheson's Hell House and Clive Barker's Books of Blood fall into the same category that Anne Rice and Stephen King's novels do.

I'm sure that there are a dozen of other great 'classic' or 'cult classic' novels and authors that I have overlooked. Therefore, I encourage a conversation : what would you classify as a 'classic' horror novel and why?