I have two. The Ritual and Drowning.
The Ritual is pure smut. 61 chapters and every one contains a graphic, demeaning sex scene. I’m convinced the author was an adult film screenwriter and decided to try their hand at writing a novel. There isn’t a single likable character in this book.
Drowning isn’t a great work of literature. I felt that the characters were very two-dimensional. I hated how naive the author made the protagonist. It made me have a lack of sympathy for her. The ending was series finale of Dexter bad.
I didn’t expect to feel this way before I read it, but the answer is definitely “Blood Meridian” for me. It was not an enjoyable read and the lack of punctuation was maddening. The overall story was good, in retrospect, but the read wasn’t enjoyable for me.