The director of this movie, Harry Potter and the Deathly Hallows: Part 1 also directed arguably the worst movie in the series, Harry Potter and the Order of the Phoenix. See, the problem with that movie was that it tried to fit the entire book into a 90 minute movie, which caused it to not make very much sense of have an overarching plot. This movie is amazing because it is an almost transcription of the book into movie form. He doesn't have to cut anything out, or squeeze it into a short movie, because there are two movies to the one book. Yes, this movie doesn't have much of a plot, but if you saw the first half of any long movie, would it? It is mostly brilliant exposition with phenomenal characters and mind-blowing effects. The score is out of this world, the composer of The Sorcerer's Stone and The Chamber of Secrets came back to create arguably the best score of the series.
I think what makes this movie one of -if not THE best movie of the Harry Potter franchise is that no sub-plot or detail had to be cut out or squeezed from the book.
Probably the reason that this is one of the best is that JK Rowling herself actually wrote the script, so everything fits and leaves the most particular Harry Potter fanatic fans leaving the theatre ultimately satisfied.