I gave this book five stars because I was completely enthralled while reading. The interactions between the characters remained completely realistic (as in all of Rowling's books). And the plot!--I've never read 700-some pages so quickly!
Although I thoroughly love Prisoner of Azkaban's plot twists and mystery, this book easily stands out to as my favorite of the series. We gain much clearer perspectives of the wizarding world--the prejudice, the fear, the Ministry's corruption--but Dumbledore, Hermoine, and Harry all present wonderfully fresh views for equality and against prejudice. Their open acceptance of giants, house-elves, and other shunned magical creatures impressed me.
There is, however, one thing which prevents me from truly giving the novel five stars (in reality, I give a 4.8 or 4.9), and that is Harry's crush on Cho Chang. Yes, she's pretty. Yes, she's an exceptionally good seeker. Yes, we all go through crushes as adolescents. But his crush frequently got in the way of trusting Cedric and pursuing steps in the Tournament. Others may disagree, but I found the crush downright annoying. But this in no way hinders my love of the book.