Given that I don't read much by anyone other than king, mine are all by him. The only books I started and didn't finish because I didn't like the book are The Regulators, and Misery. The Regulators was probably because I'd just finished desperation when I started it and probably just worn out on the characters (even though they're flipped). Misery, I shouldn't have started it to be fair, it's just something I knew I wouldn't like. I'm not the biggest fan of King's "innie's" and while Annie is horrible, Paul was a bit of a prick so I had nobody to like.
Finders keepers is easily the worst book I've ever finished.
I didn't care for Finders Keepers either. Granted, I didn't read Mr. Mercedes, so I don't know if that would've improved upon my experience with the book. For me it was sort of, bland. Certainly not one of the worst books I've ever read. I don't think anything SK has done would ever fall into that category for me. There are some that miss the mark (Deloris Claiborne is like that for me), but aren't outright terrible.
City Infernal, now that's a book that I found so awful, I had to give up about 50 pages in.