Eragon.
The Eragon book is so-so but really would have been easy to make into an okay film, basically came across as no one in it had read the book.
The Last Airbender
Avatar: The Last Airbender is amazing but got Shayamalaned.
The Golden Compass
His Dark Materials is a great series and this film did enough right that someone a) must have read the book and b) cared about getting it right. How they proceeded to make a such a series of terrible judgement calls is beyond me.
Assassin's Creed
This game series could have been used for a great film, you make either a James Bond or Indiana Jones film and set it in a historical time period. Job done. Some interesting and promising ideas just bogged down into much exposition and not enough development, that even after watching it I could only name four of the characters.