I think mine’s Z for Zachariah. The book is insane. A psychological survivalist thriller between a 14 year old girl and a strange man with unknown intent. The movie is… not this. They changed everything meaningful about the characters and lost all the tension. Hope someone does it properly one day. What’s your adaptation peeve?
The Lawnmower Man is both the winner, and kind of cheating as an answer.
The studio had an existing script called Cyber God that they had been sitting on for a while. When they finally got around to making the movie, they figured if they could stick Stephen King’s name on it, then they might get more people to come see it. So they went out and bought the rights to a random King story called The Lawnmower Man and then changed the title character’s job in their movie from janitor to gardener so he could use a lawnmower at least once.
King actually sued them in order to get his name removed from the movie and won. Then they put it back on for the VHS release anyway.
So it kind of hads to be the worst adaptation because it’s not even an adaptation at all. But also cheating a bit because not really an adaptation.