This is an interesting topic. There are so many bad adaptations that it is hard to nail a single one down. The thing that particularly bothers me is when the film betrays the source material. I can forgive the heavy editing that is necessary when one must sum up ten or twenty hours of book in a a ninety minute film. However, when the message that made the book valuable is removed, I may become upset. A lot of people really loved The Lord of The Rings and The Hobbit movies but the things that gave depth to those stories were kind of abandoned in favor of action and special effects.

Shakespeare adaptations tend to be pretty awful (there are some good ones though). Again, I think the same problem is at play. There are a lot of important things that ignored by film makers because they aren't particularly exciting.

As to the movies that legitimately made mad, I think Troy was super annoying. They took this great bit of mythology and removed the bulk of the mythology. To make it worse, they ignored everything that makes the Iliad interesting (the human story and deep characters) and gave us a paint by numbers action movie.

If we can extend this discussion to television, I would point to Game of Thrones and both an excellent and poor adaptation of a series of novels. There is a lot to like and they did a few things right but, particularly in the later seasons, they changed some extremely important things to make the show play better for a mass audience. That is fine from a business standpoint but it damages the property artistically and it often steps on its anti-war, anti-authority, and income inequality messages.

This is probably getting too long so I will stop rambling at you. Great great question though.