Harry Potter and the Goblet of Fire
In 'The Prisoner of Azkaban' Harry Potter and the reader began to see the wizarding world as the flawed human place it is rather than the whimsical escape it had provided for him before. At the start of 'Goblet of Fire' its no longer possible for summer to reset the status quo the way it used to. Potter and Voldemort's actions begin to have real repercussions in the outside world and he starts to get noticed as someone more than as "The Boy Who Lived".
Rowling introduces the international wizarding community here as well through the Quidditch World Cup and Triwizard Tournament and makes some clear commentary on the fallibility of government and sends up unscrupulous journalism with Rita Skeeter. She introduces new rifts between Harry and his friends and side characters from your various Weasleys to Hagrid to Malfoy take on a life of their own. Rowling may have always had a notebook full of notes on the lives of her other characters but its 'Goblet of Fire' where she first lets them play out alongside Harry on paper.
This was the first book I had to really wait for and anticipate and I loved it. I can still remember the rainy afternoon that I read it for the first time, curled up in an armchair and so absorbed that it took my sister snapping on the light to realize I'd been reading in near darkness for some time.
Before I end this I need to add that I love S.P.E.W. Hermione has the guts to stand up for her convictions against near-universal disdain and apathy. Harry's the hero, but Hermione is the series' conscience.
Harry Potter
