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?
What’s interesting about that is they actually filmed a more book faithful ending, but test audiences said it was too depressing, so the studio made them film the happy ending instead.