Lawyer, mother, avid reader. Game host extraordinaire! Partner in crime to Obsidian Black Plague! My bookish weaknesses include classics, fantasy, YA, and agreeing to read more books than is even remotely possible.
There's been some discussion already on what makes something "classic horror" versus "genre: horror." Bear in mind that this is but one person's interpretation of the bingo categories (albeit one who was a joint drafter of the categories) - this game is supposed to be fun, so anything that you are comfortable defending is basically fine with Obsidian Black Plague and I!
But, having said that, my personal interpretation of the categories would be that classic horror is narrower than horror genre, and that the books that would fit into the subset of classic horror need not necessarily be old, but they should have a substantial following that will argue energetically for their inclusion in the horror canon.
One place to start is with early horror and influential horror. One can do no better than to turn to those classics of horror literature chosen by Penguin to feature in their (gorgeous) Penguin Horror editions:
(From left) The Haunting of Hill House by Shirley Jackson, Frankenstein by Mary Shelley, The Raven & Other Stories by Edgar Allen Poe.
Other similar selections would include: Dracula by Bram Stoker, The Ghost Stories the Antiquary of M.R. James, The Turn of the Screw by Henry James, pretty much anything by H.P. Lovecraft, The Island of Dr. Moreau by H.G. Wells, and The Strange Case of Dr. Jekyll and Mr. Hyde by Robert Louis Stevenson. Shirley Jackson's other classic, We Have Always Lived in the Castle, would also qualify here.
If none of those appeal, there are several books by more modern authors that, IMO, could easily be defended as "classic horror," including some of Stephen King's books, including The Shining and The Stand, Anne Rice's Interview with a Vampire, Jaws by Peter Benchley, Flowers in the Attic by V.C. Andrews, Watchers by Dean Koontz, Rebecca by Daphne DuMaurier and The Stepford Wives by Ira Levin.
And, even if none of those appeal, and you're looking for something short to fill the square, there is some YA that I would submit to you (in my lawyer talk) definitely qualify as classic horror!
From left: Wait 'Till Helen Comes by Mary Downing Hahn, R.L. Stine's Goosebumps (all of them), Down a Dark Hall (or pretty much any of Duncan's other books) by Lois Duncan, and The House With A Clock In Its Walls by John Bellairs, all of which have been scaring the bejeezus out of children (including me, when I was a child) since the 1970's or '80's.
And genre: horror is pretty much everything that is left!