The Hobbit. Probably not the worst movies with not the worst bastardisation (that'd be The Dark Tower for me), but I simply can't wrap my mind around the overbloated monstrosity that the Hobbit TRILOGY is. Like why would anyone do this, it felt like it's in the bag, they got Peter Jackson, they already made LotR to great success, why do we suddenly need wacky wheels with cartoon CG goblins in 48 FPS for some reason... It doesn't even match neither the tone of the book nor the tone of LotR movies.
"I Am Legend" has been made into 3 or more movies, none of which have anything like the book's ending.
The Last Man on Earth (1964) is dull and misses the point almost entirely, but almost manages the title line. Not quite.
The Omega Man (1971) is exciting and misses the point even further.
I Am Legend (2007) almost gets it. The vampires are competent. Will Smith's smarter than Neville of the book, but crazier. But then both endings fail to treat the vampires as a society.
Not a classic book, but Artemis Fowl. Disney managed to confuse fans of the books and newcomers to the series alike by adding a McGuffin that was unnecessary, bringing the antagonist from the second book into the movie on the first book, and mangling the relations between the two main protagonists beyond recognition.
The Wheel of Time. I waited for reviews before watching it, so glad I never wasted a second of my life watching that piece of blasphemous garbage. Just stick to the source material, how fucking hard is it??? Apparently too hard for modern directors, they have to "fix" everything and make it appealing for a "modern audience." Bitch, I am the modern audience, and fuck you.
Starship troopers. I say this not because the movie is bad (it's not, I think it's exactly what it meant to be and did it well), but that the movie and the book are thematically opposites. The book is very pro military authoritarian. The movie is a satire of that.
No one appears to have yet mentioned Forrest Gump. In the book he was a chess grandmaster who wrestled professionally and was an astronaut. Also, the book sucks.
You'll probably recognize it as Blade Runner but the film took so much liberty the author allowed a good friend to write three sequels in order to harmonize the book with the movie.
American Gods: they made unnecessary changes and introduced unnecessary filler plotlines until it felt like a drag to watch. The book already explored social issues, but the showrunners decided to dial it up to 100 and spoonfeed it to the audience at the expense of the actual plot.
Ready Player One: they dumbed down the whole thing about hunting keys and portals, removed tons of important worldbuilding details, made pointless changes that ruined the spirit of the books. They should have made it into a series instead of a movie.
Oh, another one I just thought of - How to Train Your Dragon.
The movies are fine, but they are so completely different from the books in almost every respect that it's barely worth giving them the same name.
The books are absolutely brilliant, especially the further you get into them. Would love to see them developed as a TV series that stuck to the style and messages of the books. Would likely need about 10 seasons though!
I'm going to flip the spirit of the question and say that Michael Crichton's Timeline movie adaptation is so bad that it falls into so bad it's good territory. I own it on bluray, and we watch it at least once a year.
Any visual media that you've seen after you've read the source book. A better way to look at it. It is which movie was better or as good than its book.
Jurassic Park was a better movie than the book.
The Martian the movie was as good as the book.
"The NeverEnding Story" should never have been made into a movie. It's almost ironic.
Every time a child watches the movie instead of reading the book, that's an opportunity lost.
I'm sure it's not the worst but I felt like the adaptation of Watership Down changed the tone/message compared to the book. Now granted the infamous violence is present in the book (though seeing it is more visceral than reading about it). But in the book there's a nice story at the end where Hazel is injured (iirc) and is taken in by a little girl and her parents who take care of him while he recovers before releasing him back to the wild (which only adds to his legend, of course).
Removing this bit, the only positive interaction with a human, makes the message feel more like, "Humans are bastards and inherently anethma to the natural world, which is also a brutal war of all against all even down to the cutest softest creatures." It just makes you feel bad, whereas the book might make you feel bad at times but it also offers an example of what you can do right. It's kind of a pet peeve when a work with environmentalist themes falls into that line of "Humans are the problem and there's nothing you can do but feel bad about it."
The two adaptations of Watchmen have both missed the point. The Zack Snyder movie treats the characters like gods rather than deeply flawed losers and weirdos.
The HBO series is better, and does get very close, but collapses from a meandering plot and glorifying cops
Well. It's clearly not a book, but Disney's Hercules was the first time I really felt disappointed about going to the cinema. My ten year old brain was having none of it. I wanted the adultery, the murder, the dirty stuff the story was supposed to have and I think it's the Disney film (that I've watched) I hate the most up to date.
Don't know if it counts as "classic", but Mortal Engines comes to mind. The film cut out over half the book. I loved the book and got really excited for the film, but it was a massive let-down. They could've easily made the film twice as long, maybe more.
Not a book and not a movie, but that Cabinet of Curiosities series adapted a couple of HP Lovecraft stories and it was fucking terrible. There were a couple of beats that were interesting, but generally it was very faithless and the changes were for the worse. There were some excellent episodes otherwise, but I can't help but feel that they are just butcherings of much better stories that I haven't read.
Politically, it's way less bad than you'd expect, I'd recommend watching it. One of the best episodes had -- to someone as brainrotted as me -- an incredible hybridization of classic horror and battle anime logic. That one was probably my favorite one, though there was one where the protagonist looks just like the Disco Elysium guy and kind of acts like him too, and it was fun.
P.S. did you know that there are movie adaptations of Ayn Rand's drivel? If you are masochistic, they might be fun to watch
The vampiers assistent, bases on the Darren Shan series. The tried to fit the first 3 books and the last one in one movie, and skipping over the other 8 books.... And who is Rebecca the monkey girl.... I wand Debbie and Sam....
Taras bulba. the american version focuses way too much on the love story. The sets are awfull. Taras does not even get burned at the stake.
The russian verssion is better. But it still misses the point. It plays it straigth while the book is suposed to be satire. They kill his wife at the begining using it as a justification for the war, when she should die at the end as a consequence of taras being an ass.
The point of the book is to show that the myths of the russian frontiersman are bullshit, because if they really acted like taras does they would all get whiped out.
Gogol is still an arch reactionary and you can tell he geuinley beleves some of the things he is moking. So its suposed to have a silly ironic self mocking sligthly nhilist tone. Remininicent of a lot of stuff in the internet. But both adaptations play it straigth and pay too much atention to the polish girl.
I hate to admit it but it's actually worse an adaptation than the Starship Troopers "adaptation." Although admittedly I do like the JCV movie. I used to like Starship Troopers until I found out the director made a mockery of Heinlein on purpose because Verhooven is a jackass. Did you even read the book?
Anyway.
As I understand it there actually is a reason for this. Basically, a studio ends up with the rights to an IP, and they sit on it because they suck at the one thing they're supposed to be good at. Then along comes somebody with a project idea, and the studio goes, oh that's similar to something we already have in the pipe. Then they steal that idea, tweak the script to include at least one or two elements from the IP, claimants an original work and they don't have to pay the original screenwriter, and churn out something that may or may not be any good, but is nothing like the IP, thus potentially making significant profits for the executives at the meager cost of pissing off the original IPs core fan base.
The Lovely Bones. Haven't actually read the book, but that movie was a painful experience to get through. Peter Jackson knows how to do special effects and spends over two excruciating hours showing off all of them, even though they add little to the story which could have been told in less than half the time.