Some sci-fi and fantasy books just spend most of their time world-building and not as much on character. When you have a visual adaptation however you HAVE to tell the story through the characters, not descriptions. Don't get me wrong I love the following books, I just think the movies did a better job telling a story.
"Logan's Run." I love the book, the movie, and the TV series but wow are the characters flat in the book and maybe a bit dated! Just to complicate matters all three versions have a different ending. (Truth be known, while the TV series got the worst reviews it is solidly my favorite version because Logan and Jessica are beautifully wonderfully human in that version. As skilled as Micheal York and Jenny Agutter are as actors Gregory Harrison and Heather Menzies made me actually love the characters and overlook the "Dr. Who," budget and occasionally clunky plots.)
The "Lord of the Rings," books (except maybe "The Hobbit"). I know I'm also going to geek/gamer hell for this sacrilege, but as wonderful of world as Tolkin designed the movie focused more on the characters and story then the book did. The book expertly builds the world, its history, and the cultures in it, but the movie is actually a story with an amazing cast.
"Do Androids Dream of Electric Sheep"/"Blade Runner." The movie is a masterpiece. The novella is amazing, but not a masterpiece.
Special mention goes to George Lucas, who released his novel version of "Star Wars," about a month before releasing the movie. Doesn't take a much thought on which was a greater success and why.
Honorable mention also goes to Marvel comics live action movie franchise (except for "X-Men 3") and DC's *animated* movies and television shows (except "Teen Titans" up until the last couple of years). The comics have been just twisting and killing characters right and left in recent years to drive up sales through shock. The animated stuff from DC and the live-action Marvel have been almost reviving what made comics fun and readable in the 60's to mid-90's - and they tend to have a better grasp on character history.
Finally, I have to point to an even rarer instance that does happen - when a novelization of a movie is better then the movie because the author can make you linger on character moments and/or fix plot holes. I have two of these on my shelves. A.C. Crispen's "V," novelization and Alan Dean Foster's "The Last Starfighter." The "TRON," novelization bears a mention here because it actually shows cut scenes and fun moments you can lose if you're watching a "full screen," version of the film.
EDIT: My husband just point out a few more movies that are, "better then the source material." One would be the movie "Clue," and the other are the "Pirates of the Caribbean" movies.
Other book/movies mentioned were "Forrest Gump," "Sense and Sensibility," "Women in Love," and "Brokeback Mountain."
Anyone have a movie they'd like to add to this?