Harry Potter and the Prisoner of Azkaban 
I may be a little biased, since I’m an Alfonso Cuarón fanboy, but this is probably the best “Harry Potter” film. The editing’s improved, as well. Part Three is where the franchise really started to go darker and more mature. The themes and the cinematography reflect that. Emma Thompson joins the cast as Sybil Trelawney and Gary Oldman does as Sirius Black. Both amazing.
I agree this has always been my favorite of the entire series. Part 2 of Harry Potter and the Deathly Hallows might be my new favorite since it ended the whole series, but that would only be because it was the last one. Azkaban was the best story overall.
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