Martin Scorsese is very good at taking the stories of other people and adapting them to indict the authors. Wolf of Wall Street was brilliant in that, and many other ways
Jurassic Park. Just got around to reading the book last year but it's been my favorite movie since it first came out. The movie is significantly different from the book, but I think every change Spielberg made was a brilliant one.
Also, while maybe not quite as good as the book, Tom Tykwer's recent adaptation of Dave Eggers' A Hologram for the King was about as faithful an adaptation I've ever seen.
I think The Perks of Being a Wallflower was fantastic. The author was the director so that probably had something to do with it.
The 1961 version of A Raisin in the Sun is up there for me, easily, but that's almost cheating since Hansberry adapted her own work.
Have you read Wiseguy by Pileggi (basis for Goodfellas)? I read it a few years ago after seeing Goodfellas dozens of times. Considering Pileggi wrote it with Scorsese, the screenplay was an excellent adaptation. When reading the book I couldn't help but have Ray Liotta's voice in my head. Also, Zodiac (starring me)!
Upon further thought, David Lean's Great Expectations is pretty fantastic, too. Cuts out a LOT of Dickens' fluff to keep a pretty powerful story.
I haven't read Wiseguy, but I was thinking of it when I was mentioning Scorsese's adaptation work. I remember reading awhile ago that Henry Hill is apparently a pain in witness protection and continuously had to be relocated because he was always telling people he was Nicholas Pileggi and Goodfellas was about him
I think To Kill a Mockingbird and LOTR are at the top of the pack for me. Perfect adaptions of incredible books. Also, Catching Fire, regardless of my thoughts on the entire Hunger Games series, was an even better adaption of what was already the best book in the trilogy. I think the last two films hurt its lasting impact, but it's still a great film.
No Country for Old Men was pretty spot on. I think only the Coen Brothers could have handled such source material as a McCarthy novel.
1. One Flew Over the Cuckoo's Nest 2. Misery 3. The Green Mile 4. Requiem for a Dream 5. No Country for Old Men
Saying The Silence of the Lambs feels like cheating but that's my vote, nonetheless. edit: the why: Ted Tally did a fantastic job of cutting out a couple minor plot points/characters (eg, whose head it is Clarice finds in the Packard/most notably Bella Crawford) without losing any of the punch of the main story. And at this point I think the acting might not even need discussing, ha.