Every film/book combo on this list I've seen argued in both directions with equally blank incomprehension at the idea that anyone could hold the opposite view.
I've made three contributions here. Partly because there aren't that many cases where I've both read the book and seen the film, and partly because generally I tend to think that the books are better!
I'm going to keep the focus on fantasy.
#1 How to train your dragon.
I read the book to my daughter and reviewed it on Goodreads. I enjoyed it, it was a good enough read. The film though, was (in my own humble opinion) much better.
The fundamental changes made for a better story. A bunch of boys training small dragons as part of the test to qualify to become full members of their clan - is changed to - a bunch of children training to battle large dragons to help preserve their clan. Our hero then becomes a ground breaker by befriending a (very well characterised) dragon and learning how to befriend/train others (all in secret), rather than a more modest innovator who in the book learns it's better to talk to your dragon and negotiate than just to shout at them as loudly as you can.
The addition of a love interest and both girls and boys into the fighting mix also helps make the film a better experience than the book (which to my mind lacked a little heart).
#2 The Never-Ending Story
Here I also read the book to my daughter and reviewed it on Goodreads. I watched the film probably 25 years earlier.
This one was a much closer call, with the film helped by the fact that I watched it so long ago. I wasn't a big fan of the film ... it was all right. I wasn't a big fan of the book, the first half (which the film was based on) was better than the film - but the second half (which I've seen one person praise to the sky) was for me as an adult reading it out loud, a dreary, repetitive, slog. I give it to the film in this case.
#3 The Shawshank Redemption
The last of mine, and not fantasy. This was a very good short story by Stephen King ... and a brilliant film directed by Frank Darabont - the highest ranked film of all time on imdb.com!
Other film-better-than-book suggestions include:
The Princess Bride
Blade Runner vs Do Androids Dream of Electric Sheep
Die Hard vs Nothing Lasts Forever
The Green Mile
I can't comment because with the exception of The Green Mile I've seen the film but not read the book. For The Green Mile I've seen and liked both, with no clear favourite.
If anyone wants to put forward a brief case for or against in these cases then I'm happy to move it up into the blog.