This was the most interesting book in the Narnia series, at least to me since I felt it asked the most interesting questions. Even as a pretty young child, I was fascinated by the ideas of multiple worlds, doomsday and fate as looked at through Lewis's eyes. I don't think I quite realized that at the time, I just thought that it was cool. I think, however, that Lewis succeeded best with this particular allegory (it didn't help that I read these books before really being exposed to the tenets of Christianity, but okay) because it was this book that got me thinking the most about God's role in the world.
Aside from all that, I loved this book and reread it several times because of how much I liked Lewis's take on other worlds and because I thought it was a really good story. I still think so.