SPOILER ALERTS: LotR, Harry Potter & the Deathly Hallows.
My favorite adaptation from book to film is A & E's Pride and Prejudice (starring Colin Firth and Jennifer Ehle.) Every character is perfectly cast; the script is brilliant, not least because it uses Austen's own dialogue as much as possible; all the details (locations, sets, costumes, props) make a compelling tout ensemble. Second in the Jane Austen adaptation race: Ang Lee's Sense and Sensibility. Not as faithful but deeply felt and gorgeous to look at and Alan Rickman and Kate Winslet.
The Lord of the Rings. Jackson, Walsh and Boyens capture the heart of Tolkien's work while making wise cuts and changes that work hard on screen. The biggest changes are the restructuring of Two Towers and Return of the King so that the main plot lines are intercut rather than followed in separate sections and the expansion of Arwen's role. Moving Saruman's death from the (cut) "Scouring of the Shire" to the meeting at Isengard, great choice. Too bad it's only in the extended version, but why watch anything else? Many small changes sparkle: Moving Aragorn's ponderings about Eowyn from "The Houses of Healing" and putting them into an electric scene between Wormtongue and Eowyn way back in Two Towers. Overall a stunning work that both honors and adds to the original.
The last Harry Potter film is the best of the series and improves on the book in that it avoids some of the book's serious problems. Harry's unforgivably consequence-free use of Unforgivable Curses is downplayed, with barely noticeable uses of Imperio in the Gringotts scenes and NO gratuitous Crucio in the climactic sequence. The movie is also much neater in setting up the confrontation between Harry, McGonagall and Snape at Hogwarts -- having Snape call an assembly to demand that the students turn over Harry, only for Harry to step out of the crowd on his own is a moment that drew cheers from the audience at my screening. McGonagall's strength was also not undercut by silliness, like her herding animated chairs around -- the stone knights, much more effective. The after-life scenes finesse their way around Rowling's clunky explanation of why Harry's not really dead. The final battle between Harry and Voldemort is extended and has much more weight than the book version, especially in the way it cuts between Harry/Voldemort and Neville struggling to kill the last Horcrux, Nagini.
