My favorite "bad" adaptation would definitely be The Shining. It's a remarkable situation where, if someone told you the plot, you'd think it was a fairly direct adaptation, but the themes and some small nuances of characterization are different enough that the meaning behind the story changes from the book. Kubrick and King come from very different storytelling points of view so it's no surprise.
Both the book and the film are stellar, though, and should be consumed if you haven't had the chance yet.
Both the book and the film are stellar, though, and should be consumed if you haven't had the chance yet.