Michael Gambon's Dumbledore...
This to me is the weakest part of the franchise and I think a huge part of why I enjoyed the first two movies so much more than the others.
In the books, and the first two movies, Dumbledore has an omniscient presence about him. Always two steps ahead of everyone else, it's as if he knew the whole time how the story would go and end. He was calm and collected. He seemed to just have an answer for everything and anticipated everything.
Michael Gambon's Dumbledore, to me, is the antithesis of this. He is incredibly aggressive, he lets his emotions get the best of him, he seems lost and confused in many parts. To me, Gambonn brought no warmth to the character at all. I thought Richard Harris was a spot on embodiment of the Dumbledore in the books. It's such a pity he died. I think Michael Gambon was poor casting all around though.