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?
Life of Pi. Not to defend the book (apparently everyone hates it and I get it, even though I LOVED it as a child) but they left out some of the most critically important parts of the story. It’s almost pointless without the deadly algae island and the end with the interview. The movie wasn’t even entertaining in its own right, imo. Like at least Howl’s Moving Castle was beautiful and entertaining despite barely following the source material.