We'll start with the one that made me cry. Eleven years later I'm still ranty.
The Count of Monte Cristo (2002)
Apparently the movie makers were unable to deal with the complexity of Dumas's plot and the character of Edmond Dantes. The ultimate genius of this novel is that Dantes is both the hero and the villain. Much of brilliance rests in Dumas's subversion of the tragedy. You have an obsessed megalomaniac who finds redemption and doesn't come to an end of death destruction taking everyone with him. He turns back just in time to not destroy everyone including himself. He does some terrible things to people who deserve them, and some terrible things to others who don't. The book is clouded in shades of gray and Dantes is a complex multi-dimensional character. That's hard to render on the two dimensions of the screen. I get that. However, the people who made the movie clearly didn't get the whole point of the book. They took Dantes's amazing character arc and flattened it like a pancake, then reduced the amazingly complex plot to a story about a protracted love triangle (where did that come from?) gone awry with a villain who would be twirling his mustache if he had one to twirl. The plot "twist" they invented is predictable nonsense one can find in any soap opera. They also took out all my favorite characters besides Dantes, probably because those characters are essential to the plot, character arc, and redemptive themes they wanted nothing to do with. I'M STILL ANGRY. This is one of my top favorite books of all time and they BUTCHERED IT.
Les Miserables (1998)
And speaking of cutting characters and plot...I still don't understand how you can make a movie of this book and feel like Eponine is an extraneous character who can be cut completely in order to focus more on Cosette. I know there are plenty of people who hate the musical version BUT at least the people who created it understood Hugo's themes and point. Same can't be said of the people who made this disastrous version. Some stories are just too epically big for a two hour movie, so maybe you should just not even attempt it.
Peter Jackson managed to avoid the mistake of cutting an epic story into too short a film version with The Lord of the Rings turning it into three wonderful movies. Then he lost all sense of reason and decided to err on the other side of the equation with The Hobbit, making a delightful adventure tale that could have been told in one two-three hour movie and dragging it out forever. Hot dwarfs aside, this movie is a hot mess. It shouldn't be because they scored on the acting talent side. Man does it drag on. And on. And on. Genevieve Valentine said everything I thought about this movie brilliantly here.
The Hunchback of Notre Dame (1996)
Is it even fair to include this? It's Disney so OF COURSE they messed it up. Except they shouldn't even have touched it in the first place. It is obviously NOT Disney material, and should not be Disneyfied. Still I wasn't quite prepared for how badly they messed it up.
Harry Potter and the Half Blood Prince (2009)
I have a love/hate relationship with the Harry Potter movies, but this one falls fully into the hate column. I can't even watch it. I felt like they focused on all the wrong things, to the exclusion of important stuff that in turn made movies 7 & 8 awkward in parts. The fact that someone was even able to MAKE this video says it all (but I'm glad thewlis rox did, because it is so delightfully illustrates my point):
Prince Caspian (2008)
Some books just shouldn't be made into movies and this is probably one of them. Prince Caspian doesn't have a lot of action. There is a lot of walking and a lot of talking. The movie producers decided it needed to be spiced up. I get that. Prince Caspian is my least favorite of the Narnia books, because it is rather dull. I simply object to the sort of spicing up they chose to do. They had everyone acting completely out of character. And yes. The unmentionable thing that happened at the end made me the most angry. Because EVERYTHING MUST HAVE ROMANCE. Ick.
Pride and Prejudice (2005)
Going to be completely honest here: I've never made it through this entire movie. I just can't. Keira Knightley as Elizabeth Bennet has me turning it off to watch the 1995 BBC version every time. I've never made it past the first 15 minutes.
ETA: The Tale of Despereaux (2008)
I can't believe I forgot this one originally, but Under a Gray Sky's post reminded me. And it so deserves to be counted amongst the worst. Bit hated it. And she was only four at the time.
Another one I can't believe I forgot! Escape Through the Pages reminded me of this one. I think it is that I have actively tried to erase this movie completely from my memory. It was such an awful distortion of the book. Shudder.