The sixth book: Harry Potter and the Half-Blood Prince
Definitely another tear jerker! One of my favorites because you learn so much more in this book – more about Voldemort and the various teachers at Hogwarts. Things are definitely heading to a head and it sets the scene very well for the 7th and final book.
Even though this book is one of the smaller books (168,923 words as opposed to 190,637 in the previous book), J.K. Rowling manages to pack a lot into this book.