Prisoner of Azkaban, duh. I say this because it includes the best use of time travel I've ever seen in fiction. It also includes one of the best plot twists of all time (Scabbers is actually the man who betrayed Harry's parents what?).
PYN to be tagged. I will finish this, it might not end up being 30 consecutive days, but it will get done.
Day 1 - Favorite Book in the Series