Defiantly Percy Jackson & the Olympians:The Lightning Thief. I loved the first book and have read it multiple times. They changed part of the plot, and that pissed me off.
Percy and his mother were supposed to go on a vacation, and that never happens. Speaking of his mother, I didn't like the actor they chose for her.
I also hate how they changed the appearances of the characters. Percy was supposed to have black hair and green eyes, although Logan Lerman didn't do a bad performance. His acting was fine.
Grover's performance was pretty good, and I'm not going to say anything of his appearance, even though I don't think his character was supposed to be black.
And then there is Annabeth. I freaked out when I saw she had brown hair and blue eyes, and not blond hair and grey eyes, because Percy emphasized about her having grey eyes when he meets her.
Now I know movie's have to cut some things from the book, but I don't like plot changes. For one, in the book it's almost the end of Percy's school year at Yancy's, and Yancy is a boarding school, not a public one. Which reminds me, they're only supposed to be twelve, not sixteen. They completly forgot about Hade's Healm of Darkness or whatever it's called, and he isn't an antagonist. Grover isn't left in the Underworld, Percy's mother is, and Percy doesn't encounter Enchida and the Chimera in the first book, causing Percy to jump off some building into the river where he gets the pearls.
And Ares isn't even in the movie.
Sorry for ranting, and I've probably missed stuff about characters and plot, but this movie pisses me off.