@gembo, I actually don't know... Though, ummm, if books read and re-read and re-re-read etc. etc. as a child count, I suspect it might be The Hobbit. (Special mention for an amazing text titled Poison in Animals that I insited on borrowing repeatedly from the library. Non-fiction though, so not in the running.)
Meanwhile, I have managed to get to 43 earth years without ever reading The Adventures of Huckleberry Finn and now I am I'm kicking myself for being so late to it. It's so much better than the iffy TV adaptations I saw growing up!