Excellent choices, Bren, Ninja and Wakko.
Excellent question, too. I know what my favorite book is, but series... Perhaps mine is Narnia as well. But if not that, either Redwall or Eragon. But now it comes to my mind that probably my favorite series are what I call the "Billy and Bonnie books." The Dragons in Our Midst series and the sequels. Yes, now I've finally made up my mind. Those are very contemporary and dear to my heart, as good as close to a perfection of all the modern world has learned about the sleekness of writing and exciting the reader as can be found.