I haven't read the Harry Potter series in a long time (I'm rectifying that this November) so it's hard for me to distinguish which is my favorite but I definitely preferred the earlier books to the later ones. My favorite is among the first four.
It's hard to choose a favorite among this series, but the third book, The Treasure Map of Boys, wins by a nose. Ruby has the most growth in this book, and finally learns some important lessons that she struggled with during the first two.
A Song of Ice and Fire:
Book three, A Storm of Swords, is by far my favorite book in this series. As I said in my very short review, even if the rest of the series sucks reading it will have been worth it just because of book three. I don't think I've ever exclaimed "What!?" so many times while reading a book before.