Harry Potter & the Half-Blood Prince.
I know there was a lot done wrong with this movie. A lot of great things were left out from the book that I would've loved to see and all, but... I don't find this movie that bad. Admittedly, I found certain plot points that were left out or changed in Goblet of Fire & Deathly Hallows Pt2 annoying & I absolutely hated how Order of the Phoenix was done (it was my favorite of the series until Deathly Hallows came out) That being said, so many people were looking forward to this one & I was looking at it realistically. After the changes made in 4 & 5 did you really think they would stop there? I was surprised as hell when Deathly Hallows Pt1 turned out as close to the book as it did.
Basically, a lot of what they added was really great to see & I loved the atmosphere of this movie where it tried to say "**** if everything's gone to ****, but there's still a ray of hope," only to have that 'ray' shattered with Dumbledore's death at the end. Is it perfect? Hell no! Is it underrated? I'd say yes.