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?
Oh, also, basically every adaptation of a Michael Crichton novel other than Jurassic Park. Jurassic Park wasn’t exactly faithful, but it’s an entertaining movie if you take it on its own terms.
Eaters of the Dead is a quirky faux-academic tale complete with fake footnotes, but 1999’s The 13th Warrior lost all of that tone and felt like a standard action flick.
And 1998’s Sphere was pretty forgettable.