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 Maze Runner series, specifically The Scorch Trials. The first movie changed a lot (the WCKD instead of WICKED really irked me) but the second movie completely deviated from the plot of the second book, it was unrecognizable.