Going to have to second The Dark Tower. To say it was a letdown is nowhere near enough.
The Witcher show starts off pretty well but quickly gets worse and worse. That's probably my number two.
I also thought The Hitchhiker's Guide to the Galaxy movie was pretty disappointing, though not the worst of the worst.
I could probably think of a lot more if I browsed my book collection. Rare is the adaptation that meets the quality of the book. That would be a much shorter list. If we were looking at that question, the first movie that comes to mind is The Amityville Horror because that book had some of the worst writing that I have ever subjected myself to.