The ghost stories of Edith Wharton and The Turn of the Screw by Henry James remain among the most read writings by those authors. Robert W. Chambers was prolific but is primarily remembered for his weird writings. Dracula remains popular, in part because of films and the stage play while other works by Stoker mostly languish.
Which is to say that King will probably be read 50+ years from now, but right now it's a crap shoot to choose which ones. My guess is Salem's Lot, The Shining and Pet Semetary. These three held the most visceral impact on their first audience, some of which is communicated in tha audience's writings -- fiction, reviews, commentary, critiques -- and the first two had successful filming. If the second IT movie is successful -- the first one was monetarily, I think; artistically, so-so in spite of good performances all around -- then it, too, may hang on to the long-term public imagination.
Baylor, I get your point, but since Dracula, The Strange Case of Dr. Jekyll and Mr. Hyde and others have been adapted to various later formats (radio, movie, tv, ...) , I don't see a reason to suppose at least some of King's work won't make that transition.
Randy M.