The Deathly hallows, because it sort of clears up everything...it explains all the damn reason for everything that has been going on in Harry's life...and we get to know everything about Dumbledore too..

It is a tie between Half-Blood Prince and Prisoner of Azkaban

"harry blows up aunt marge, the dementors come and take charge, lupin is a wolf, the rats a man, and they use time travel so they can save the prisoner of Azkaban who just so happens to be Harry's godfather, I don't really get it either" - harry potter in 99 seconds, the prisoner of Azkaban verse.

That's why PoA is my favorite book :>

PoA

Okay... so Prisoner of Azkaban is by FAR my favorite... But people over look how many dope and savage lines all these angsty teenagers have in Order of the Pheonix!

*ESPECIALLY All the Weasley's!*

Goblet of Fire it is the start of something bigger.