HP3 (also HP1 and HP2)

HP3 is definitely the most coherent book in the series. Everything fits. There's huballoo in the beginning over a convict, Black, amongst Muggles and wizards alike. Padfoot appears, and McGonagall's first lesson is about the Animagi. Hermione's timetable require a time turner. Hagrid's Hippogriff in the first lesson is a major part of the plot. Crookshanks and Scabbers... of course, and Ron and Hermione's fight is the natural consequence. Harry hears his parents for the first time, and the story is all about his dad's friendships. The Marauder's Map is all about his dad's friends. And everything, everything is resolved in the end.

There isn't a single extraneous element in the story. Everything works together.

To add to that, Lupin is one of JKR's best creations I think - someone who is so likeable, kind, and confident. There's a happy beginning with the sunny days in Diagon Alley, and a happy ending. Easily the most heartwarming book.

In the other books so far, I think the Dobby storyline is extraneous to HP2, and HP1 I never "get" because there's so much going on. The Dragon incident is a climax in itself, but it's not the real climax and takes away from it. The unicorn and the Forbidden Forest fiasco takes away from the climax, too, I think. HP1's ending disappoints me because Harry is simply physically saved from Quirrell by Dumbledore, and a pile of housepoints are awarded to an incident which the rest of the school hasn't even witnessed. HP1 has a strong exposition, I think, but the ending is too glossed over. HP2 is more coherent than HP1, and would be perfect if there was no Dobby - although then I guess Dobby would have to wait until Book 4.