It's true, that. I get touchy when people tell me how original a film is, and I am forced to point out that it is a remake of a Chinese black and white film or an adaptation of an obscure, rather bad book from twenty decades prior. I know my adaptations, and I love them. And, perhaps coming with the love of film, I am very forgiving of adaptations. I accept the limitations and abilities of film over other mediums, be that television or the written word. And I have seen it done well enough times to know that there is no excuse for a bad adaptation.
My favourite book-movie adaptation would probably have to be Jaws. Jaws is a flawless film, the book is... not. As an adaptation, I wouldn't call it great. If we're just talking about how well it conveys the same information in the new medium, yeah, it changes things. But it makes it much, much better than the book ever did. It's honestly a pretty terrible book. Things that I would rank in a similar category with this would be Silence of the Lambs, The Godfather, etc. Where the movie is near inarguably superior to the book it is based on. If you'd just like something that adapts it with few changes and does it well, I guess I would go with Fight Club. Even the author admits to that one.
And, since I feel it should be addressed given the Potter lovers in the group, I would say that of all of the Harry Potter films, my favourites were probably the first two. Though not as well acted or as nice looking, I felt like - as adaptations - they were the best. They didn't keep all of the content, but more than the others, they kept the feeling, and I think that's the most important thing. I don't really care how they got rid of Norbert or if we see Snape's contribution to the protection of the Stone, I just want to feel the same way I did reading the books.
My least favourite book-movie adaptation would be Eragon. I preface this by saying that I don't like Eragon, but my problems with it aren't really in regards to storytelling. For all of its problems, it isn't incoherent nonsense, which is what the film is. The film version of Eragon skips very large, very important chunks of the novel in favour of... nothing, absolutely nothing. I feel like the director took seven chapters at random and chose to adapt those and nothing else. It's a bit troubling, honestly. A similar instance of this would be Everything is Illuminated, another book I didn't really like but oh god did they ruin it with the movie. That, however, was because of the book. I would put the book alongside House of Leaves in terms of how much the narrative itself is important to the book, and a film would have to work hard to come up with something similar. And they just didn't try.
For Harry Potter, honestly, movies 3 through 5 all left a bad taste in my mouth. I love Half-Blood Prince, and the first half of Deathly Hallows, but... Prisoner of Azkaban was a jumbled up mess that I really couldn't appreciate, and that still it ends on (the one with Harry on the firebolt) just made me more annoyed when the credits came around. Goblet of Fire was just... confusing, honestly. Reminds me of The Last Airbender (though not based on a book, I should point out now that this was the worst adaptation ever made of anything in history). And Order of the Phoenix, more than any of the others, broke the rule I mentioned above. It didn't feel like the book, at all. I don't care about the changes, I care about the tone, and I just felt that it was really off. And it didn't improve anything with it. HBP was a bit off in tone, but I liked it. Same with Deathly Hallows - I honestly like the movie more than I like the first half of the book, because at least it wasn't boring.