Harry Potter and the Prisoner of Azkaban
The third book in the Harry Potter series achieves the best balance between the central plot, subplots and the overall story arc. It’s also the first book where it really feels like Rowling’s world becomes hers, rather than a tasteful mix of elements from earlier children’s literature.
Finally the paranoia and danger of the wider wizarding world begins to seep through. Things happen that will have real repercussions for the rest of the story arc. The characters start to grow and you can see the will become more than the archetypes the were at the outset of the series.
From here on in the books become less tightly structured and have more flaws but the world and the story arc only become more interesting.