The obvious:
The Godfather and The Godfather, Part II. I really like the novel but those films are spectacular.
I also agree with those voting for The Lord of the Rings. The only problem I have with the novel is that Tolkien's technical writing ability is exactly what you'd expect of an interwar professor of philology - i.e., not much chop.
The obscure:
The Devil's Advocate, which is literally my favourite film of all time and the first acting performance of Keanu Reeves' screen career. Andrew Niederman's book is interesting but the film, which takes little other than its central premise from the novel, is far superior. Full credit to Johnathan Lemkin for taking a decent idea and turning it into a spectacular one.
(The reason it's my favourite film is simple: I wrote my Honours thesis on representations of the Devil in cinema, and I discovered that I can watch it over and over again without getting bored. It's not the only film with this property for me, but it's got such an excellent premise.)
A Time To Kill - There are so many excellent actors in this film, they lift it above the fairly mediocre John Grisham potboiler it's based on.