Frederica Mathewes-Green thinks that Prince Caspian is a much better film than a book. There is also a list of other films for which many think this the case.
But doesn't it matter (and quite a lot) whether one reads the book, or sees the movie first? If you like either a book or a movie when you first experience it, it seems more likely to me that you'll be disappointed when you do the other, because it may not meet your expectations, or have the features that you liked.
I've decided that book and film are different interpretations of the same story. Most books are so intricate, you'd be pressed to make a good movie out of them, so I go to see what they needed to change.
When I was much younger, Catch 22 was absolutely my favorite book. I'd read it many times by the time the movie came out so of course I rushed to the theatre.
The movie was rubbish. Not only had they taken unforgivable liberties with the story, but the casting was universaly bad, from Garfunkle and Voight on down.
Many years later, (and many years since I'd last read the book), I saw the movie version on TV. Interestingly, I found it quite a good movie.
It was no masterpiece, not even a flawed one, and Garfunkle still couldn't act to save his life, but because I'd lost contact with the storyline and the novel's characters were no longer living in my mind, I could appreciate the movie as a movie.
For me at least, the last part of the equation is the most important. When you see a movie of a well loved book and you know your favorite character doesn't look like that, talk like that or act like that then the movie hasn't got a chance of hooking you.
Most of the Hitchcock films would probably fall in the category of improving on the source material (mostly short stories, in Hitchcock's case.)
A few of the Bond films also improve upon Ian Fleming's original work. "Goldfinger" was case-in-point, turning a decent adventure story into something unforgettable and iconic. The 2006 version of "Casino Royale" was a great film on its own, but it faithfully captured the spirit of the novel while adding enough content to make it suitable for a big-screen epic.
A much smaller category is "TV shows that are better than the movie they're based on." I can think of two: "Buffy the Vampire Slayer" and "Stargate SG-1."
I think this piece shows that there are a lot of people who, while they can read, can not really appreciate literature. Mathewes-Green mentions more than once that her correspondents say that a movie is less intricate than the book it is based on and suggest that this contributes to its superiority. To me, this just means that these people are not capable appreciating the subtlety and richness of the written story. It is not a matter of what you have read or seen first, it is a matter of what you can take a way from the story. I had seen Gone With the Wind several times before I read the book, at the age of thirteen. Even at that age I found the book much superior. I will however, offer two movies that I think are superior to their books, Planet of the Apes (the 1968 version) and Bridge on the River Kwai, interestingly enough, both by the same author.