The first question is already impossible to answer:
What is your favourite Harry Potter book.
This is like choosing between your children. Of course you have a favourite, but you also know you can never say it out loud.
I would have to say my favourites are Prisoner of Azkaban because Harry finds out more about his parents and Lupin and Sirius are introduced; the Half-Blood Prince because Harry finds out more about Voldemort and the last one because it wraps up everything so nicely and the last battle is just amazingly written.