Surprised nobody has mentioned The Hobbit trilogy yet. Jesus Christ, it's one book and it got THREE movies. Compared to one movie per book for The Lord of the Rings which, asides from some weird crossover of events between the latter two movies and dreadful treatment of Saruman and Eowyn, weren't too bad. But The Hobbit trilogy...that went on and on and ON. It was like watching an anime filler arc movie compilation. I can't remember the last time I've been so bored watching films. Also, that they called the final film "The Battle of the Five Armies" and not "There and Back Again" was just plain stupid. Sorry, but there it is.
I'm going to echo sentiments about the Harry Potter series here too, particularly the last two - for all of the events that were actually in the book across those two films, you'd have a single two-hour film. And Michael Gambon is by far the worst actor they could have picked to replace Richard Harris as Dumbledore...the man clearly has no idea what the word "calmly" means. I will admit that part of this is bias because none of the characters looked or sounded the way I had imagined them to, and that's a hard thing to deal with as a kid, but they just missed out too many things from the books to be what I would call faithful adaptations.
Ghibli's adaptation of Tales from Earthsea is about as far removed from the source material as it is possible to get, too. It was an OK movie, but the books were significantly better so I'd say it's disappointing that it kinda just flung them aside in that regard.
Same goes for Disney's The Black Cauldron. As a kid that movie had the second most terrifying antagonist in an animated movie I'd seen - The Horned King being beaten by a mile by Watership Down's General Woundwort - but when I read the books that inspired it as a teenager, I was left very puzzled as to what the hell Disney were thinking.
I've mentioned these elsewhere, but DC's animated adaptations of The Death of Superman and The Killing Joke left a lot to be desired. Both of them suffered from the same issue, too: padding. The Killing Joke got the worst of it, as they decided to add in a Batman/Batgirl romance subplot where she gets so frustrated with her inability to remain emotionally detached she decides to make love to her mentor on a rooftop - yes, really - but neither made the transition from printed media to animation as well as they should have. Especially since they somehow roped Mark Hamill in to reprise his role as the Joker. In one of the most iconic Batman comics of all time. It's a "how the fuck did you mess that up?!" sort of thing.
Anyone seen The Golden Compass? If you have, I don't need to go into any further detail. If you haven't, be grateful that I'm going to spare you by not going into further detail.
Also - remember it was a manga first, so this counts - anyone ever seen Dragonball Evolution? If you thought The Last Airbender was bad, let me tell you, things can get worse. Much worse.
...also I'm just going to throw this out there, but every MCU movie counts as this too, because there isn't a single original idea in the entire franchise.
