A movie's already been made of Dune. It was quite good, but I did have to explain it to my boyfriend, who hadn't read it, in some spots. I think it was made by David Lynch, correct me if I'm wrong.
Hmm, I'd have to say that Holes was a pretty good adaptation (so it's a kid's book, it's still a good adaptation of it). And so was The Outsiders, actually the most accurate/good one I've seen that managed to capture the spirit of the book.
But out of books that I've read that I would hate to see turned into a movie? Uhm...
The Gone series maybe. A lot of it is based on the character's thoughts and it's extremely plotty. And lots of characters and lots of books would mean it would work only as a TV series(even then...) if you cut out some of the characters. And the study of teenagers and children's behavior. And most of the plot. Basically anything that made it interesting is probably too complicated to seriously delve into in live-action.
Other than that, I'm not sure. It really depends on what you're willing to adjust/sacrifice to make the switch between book and film as painless as possible. I'm trying to adapt a story I've written to comic form, and it's hard to get the emotions to translate properly. And, say, if I was going to make a flash animation or something, I would have different challenges.