For me it is the Harry Potter movies.
The books are all kinds of awesome, not because JKR is a great writer or anything, but because she created an immersive and captivating world in those books with memorable and sympathetic characters. Well except for Ginny who she turned into a one dimensional Mary Sue that everyone was supposed to love and think was the next coming of Lily (incest anyone?) when really she devolved into a bullying toerag who needed a slap in book 6.
Ahem, sorry back on topic.
The movies themselves... Just aren't great.
Daniel Radcliff, and Emma Watson just can not act. Neither can Tom Felton, or the guy that plays Neville, or the girl that plays Ginny. Rupert Grint was the only one who seemed to have actually learned anything or gained any acting ability.
Actually Daniel was an alright actor when he is just conversing, but the moment he is asked to Emote in any realistic way he just fails at it. He can not express emotion realistically and convincingly and just comes off fake.
Even the adult actors in these movies deliver spotty perfomances. And we KNOW that the people who play Snape, Wormtail, Bellatrix, McGonagall, Dumbledore, and Mr n Mrs Weasley are all great actors and actresses. Which of course points to exceedingly poor direction and editing throughout the movies.
They also made the mistake of starting to make the movies too soon after the books began coming out, I think they had almost caught up with the books by the time they were filming movie 5.
This resulted in them cutting things from the earlier books that seemed innocuous and unimportant in order to fit the books in, that later developed into fairly important if not critical plot points in later books; which meant by the time they hit book 7 they had to split it into 2 parts so they could go over a lot of the stuff we should have learned in movies 4, 5, and 6 that they had cut out.
The movies were all critically acclaimed, and IMO they were all schizophrenic messes that you could just tell had been cut to ribbons in the editing room. Prison of Azkaban being the worst of the movies for that.
The weren't the "worst" movies I have ever seen; but they were definately the biggest disappointment for me. They really only "got it right" in the very last movie, Deathly Hallows part two, which is a real shame.