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?
I Am Legend. The book is dark, brooding, heavy, and psychological. The movie really Hollywood’d it up, completely changing the entire point of the ending.