There is no definitive answer to this question, as King's novels can be quite dark and suspenseful. However, some readers may consider the following novels to be his darkest works:
-The Shining
-The Stand
-Carrie
-Salem's Lot
These novels all feature elements of horror and the supernatural, and deal with some pretty dark themes. If you're looking for a truly suspenseful and scary read, then any of these novels would be a good choice.