For me, Harry Potter and the Half Blood Prince. I think the book was done really well, and the Horcruxes are obviously hugely important to the plot (despite the fact we only hear about them for the first time in the second to last book), but in the film nothing really happens.
The world admits Harry was right, he’s grieving for Sirius, all the lighting is dark and dingy (yes I know the themes get darker as it goes on) – it’s just all a bit, well, grim.
And then, Ron is busy snogging Lav-Lav’s face off, Harry is on a mission to destroy Draco Malfoy, and the Death Eaters are finally back.
Something about it just doesn’t quite sit right, and I certainly didn’t enjoy it as much as the others. In fact, I think it’s the only one I didn’t see in the cinema…