Can't really decide which is my favourite book, I'm leaning toward book 6, but not overly sure why exactly, might have to get back to you on that one...
I'm going to pick 2 characters instead of 1 for favourite: the Weasley twins! Surely no explanation is needed, they are just pure awesome, with their best moment being the havoc the wreak on Umbridge!
My favourite part of the series is in book 3, during the bit where Harry and Hermione use the Time Turner, specifically when Harry saves himself and Sirius from all the Dementors with a single Patronus Charm. Always found that part to be truely amazing.
I disliked some of the plot in book 5, like how the trap could have been avoided if Harry had done things a little differently, such as actually remembering that Sirius gave him a present that he never opened, as it would have done precisely what he needed: contact Sirius.