Assuming that 'A Little Princess' is a shiny new book you haven't read yet, go with that. It is simply one of the all-time best children's books ever written. I grew up reading it and even as an adult, it's still a book I love to read from time to time.

The Lightning Thief is a really fun read as well. I always loved Greek Mythology, so taking it into the modern world made it a very fun book to read. Yes, there were a few things I didn't like about it but given that the Greek gods were pretty much like that in the myths, it's not something I'm going to complain about too much.