Now I like this question. One option would be to go for an adaptation that radically changed the source novel like Blade Runner or Minority Report. Or I could look for a more faithful adaptation like Dune or Fahrenheit 451. Then there is another category, films which made an appalling travesty out of great books like the Will Smith I, Robot.
Having considered all that I'm going to return to an earlier theme on this blog. Like most other geeky children of the seventies I was Planet of the Apes mad at the time. After being shocked by the dramatic conclusion of the original film I sought out the original novel Monkey Planet by the French writer Pierre Boule. This was quite a different planet to the one shown in the film. In the book the Apes are very civilised and technologically advanced, and the differences between the Apes and the stranded astronaut are intellectual and philosophical.
So this is a film that, in my humble opinion, improves upon the source material. The best adaptation of a book is the original Planet of the Apes movie.