The first one which comes to mind is the Godfather.
The movies are far superior to the book which seem to be based on a late night viewing of Kirk Douglas movie the Brotherhood coupled with a bizarre subplot about a woman who has a giant vagina, which needs fixin'.
Movies are too short to do a good book justice, they have a better chance with short stories.
I'd also say that the tv series Justified is better than the Elmore Leonard books, mostly because they had time to develop the characters more and because Elmore Leonard is a bit of a natural screenwriter. His spare style begs to be fleshed out by the six dozen people who work to make a good tv series, a good tv series.
To some extent, I'd say the tv series the Wire was better than Richard Price's Clockers, despite this book being a revelation, once again, because of the time they had to develop stories beyond the original work. (Anyone who has read this book will recognize that while not the direct basis for the series, being set in an entirely different city, it is obvious that the majority of the series is based on his work, despite the crap the show's other creators have claimed during interviews)