Book-to-film adaptations fail most often when they mistake fidelity for quality. The goal is not to translate every plot point onto screen — it's to find the film that captures what the book was actually about. These are sometimes very different projects.
The adaptation I think succeeds best is Arrival (from Ted Chiang's 'Story of Your Life'). It transforms a fairly spare, intellectual short story about linguistic determinism and grief into a film that is also spare and intellectual but uses visual language to do things the prose can't. The film is not a translation. It's a response.
The adaptation that intrigues me most is Dune: Part One and Part Two. Villeneuve has made choices — compressing characters, shifting emphasis — that I initially resisted and have come to appreciate. The films feel like they were made by someone who loved the book and understood that love requires judgment, not mimicry.