The Lightning Thief (Percy Jackson and the Olympians) and the 2nd half of the 7th Harry Potter or the 8th movie. The former had so many difference from the book that it took away almost everything that made the first book great and the latter was just anti-climactic. I mean, you would expect some great battle between Harry and Voldemort but it was just sad, heck the battle between Harry and Draco in the second movie was better.
P.S.
No offence intended and it's just my opinion.
P.P.S.
Sorry for the horrible grammar