The first book written, you're perhaps the most well-known of the Chronicles. From Mr. Tumnus to Turkish Delight, statue people to Aslan's resurrection, there's nothing not to like here.
Find out which Chronicles of Narnia book you are.
...if only because it was actually the first proper book I ever read, when I was five, and I really, really love it. It's got good scary bits (the bit where Lucy realises that Mr Tumnus isn't all that he seems is still genuinely shivery), good joyful bits, and it feeds into a universal fantasy - the desire to discover something new and wonderful in a familiar, ordinary setting. Hands up who spent their childhood climbing into wardrobes? And being convinced each time that this time, you should really have touched the back by now, so maybe....
I know C.S. Lewis's Narnia books have been attacked in recnet years by the dour likes of Philip Pullman for being a Christian allegory and all that, but apart from The Last Battle, which no one seems to like anyway, the Christian stuff - which, by the way, I didn't notice until somebody actually told me, even though I was quite a religious small Catholic child - doesn't get in the way of a good story. I find Pullman to be profoundly joyless, and his books hammer their anti-relgion message home with a clumsy mallet; Lewis's books, the last one excepted, are more subtle, and, ironically enough, much less didactic.