I don't know about best film to movie - of the few recent films I've seen, I thought that Harry Potter actually was significantly better than the book. That's not knocking the book which was enjoyable enough, and thankfully it was not written above the heads of the children it was aimed at, or making arch jokes for the parents. Just the film was better.

The best audio-visual medium for long books is the old BBC TV serialisation. Gives room for the characters to stretch and grow. But for a story on an epic scale you need the big screen, and the economics gets in the way, I suppose.