Rachel A. Brune
I had a new favorite book every week, so it's hard to say, but when I was in the fourth grade, someone gave me a large hardcover book of folktales from around the world. I enjoyed reading the tales and learning the recurring themes from each of the countries/geographic areas.
Additionally, I was a big Nancy Drew fan, and read every book in the series, multiple times. Speaking of series, I also read as many Trixie Belden and Beverly Gray books as I could find.
It's hard to have a favorite book when your favorite activity is reading! :D