I know a lot of people have said this, but Prisoner Of Azkaban is my favorite Harry Potter book. I just love the storyline, and so many things that happen in the rest of the series are set in motion in this book. 
 We get to meet Sirius, Lupin and Pettigrew in this book, and they are a connection to Harry's parents because they were all best friends. Until Pettigrew turned traitor and gave up James and Lily to Voldemort. 
 There are so many things I love about Prisoner Of Azkaban- the Marauder's Map (and learning who the Marauders are!), the time turners, Hagrid teaching Care Of Magical Creatures, and the introduction of Trelawney. 
 Random Movie Sidenote: I loved Emma Thompson as Trelawney. 
 Back to the book, though. You really start to see a lot of the characters start to grow up- or even see them in a different way. It's no wonder Snape didn't trust Lupin, because of the prank that Sirius did. 
 Overall, it gets a 5 out of 5. It's my favorite, and one of the strongest books in the series.