PoA, OoTP & HBP haven been my favorite books.

PoA my favorite moment is when Harry and Hermione free Sirius, and he escapes on Buckbeak!

OoTP is when Umbridge gets her butt Kicked in the Foribben Forest!

HBP is when Harry takes the Felix Felicis potion and he, Hagrid & Slughorn bury Aragog.

But my absolute favorite is from SS - when Harry learns he is a wizzard & he visits Diagon Alley for the first time.