Greg Cox wrote:
It's possible to make a perfectly faithful adaptation that doesn't work at all as a film, or to take major liberties and still come up with a great movie or TV show.
I think the Harry Potter films are a good example. The first two films are pretty slavish to the surface content of the books, but to me they fail to embody the books' spirit and tone. The third and subsequent films take more liberties with the plot and details of the books (though still hewing pretty close), and come closer to capturing their spirit.
I think that it's an essential ambiguity in the OP question as phrased. Instead of asking for the "best film adaptation", to clarify, one has to, say, ask for "best film that's an adaptation" or "best adaptation of a book into film", and put the adjective "best" directly against the word you want it to modify and especially don't fuse "best", "film", and "adaptation" into the same noun phrase!
But my point is that such qualifications aren't needed, because the literal meaning of the word "adaptation" is "change" -- specifically, change that serves a constructive purpose when adjusting to a new context. It should be axiomatic that a film adaptation will make changes from the book. The question, then, is whether its changes work well, whether it manages to preserve what's important or add something new and worthwhile. Even those first two slavish Harry Potter films changed things, but they changed them in a way that I felt made them dull and prosaic, stripping away their sense of wonder (like having the Hogwarts stairways visibly rotate, as opposed to the books' description of having the corridors change topography in an unseen, inexplicable way that was far more magical). That was a change that took things away, whereas a good change -- a good adaptation -- will be one that adds something new or makes something work better.