Wow. This is really tough!
Either Philosophers/Sorcerer's Stone, Goblet of Fire, or Deathly Hallows.
Order of the Phoenix depressed me so much xD The movie made it seem a lot more hopeful.
Sorcerer's Stone is the easiest for me to reread, Goblet is just so action packed and awesome, and Deathly Hallows is just... Deathly Hallows!
My goodness, I like them all! Dealthy Hallows is just in a realm of it's own, so besides that, I'll have to pick...oh, I just can't choose. They're all awesome in their own way.
And once again, can't you PLEASE make one thread and discuss this all there?
I started reading the: Harry Potter and the Prisoner of Askaban, I really should pick that one up sometime again, I just love that movie so much.. and the book is good, too.