I don’t know if it’s the worst, but I am very disappointed with the movie adaptation of Mortal Engines. The series has such a rich world to explore and very good plot points that would have been amazing to see on the big screen.
The movie ruined any possibility to see a sequel or even a reboot in a very long time (similar to what happened with His Dark Materials), although the fandom now prefers that if there is another attempt at an adaptation it has to be a TV series and animated.
Which books have the worst video adaptation?
For me it’s definitely the Dark Tower, but the Golden Compas was also a huge letdown.