I actually voted for the first "proper" book I ever read, which was The Lion, the Witch and the Wardrobe. I read it the Easter I was five, in my great-aunts thatched holiday cottage in Skerries, and I adored it instantly. I agree with its Big Read advocate, the non-Christian Roni Acona, who said she gets pissed off when people bash it for its Christianity because it's an allegory, and one which I definitely don't think overshadows the book. I mean, most mythologies have a resurrection myth, and just because Lewis is drawing from the Christian version of that idea doesn't make the book propaganda. I'd have more of a problem, on a purely literary level, with Philip Pullman's heavy handed moralising. Actually, Pullman's Lewis-bashing annoys me a lot, not least because Lewis more or less invented the genre in which Pullman is working. And yeah, yeah, yeah, I know, there's The Last Battle, which is terribly dodgy, but who likes that one? No one, that's who. I certainly never did as a kid. That doesn't take away from the other books.
I had absolutely no idea The Lion, the Witch and the Wardrobe was meant to be a Christian allegory for years, and when I found out, I was just really impressed by the cleverness of it (as I saw it), because it was the first time I'd encountered an allegory, and I though the whole idea was really cool. I'm rather bewildered when people say they felt "betrayed" when they found out it was a Christian allegory after reading it and loving it for years, because I certainly wouldn't feeI - and haven't felt - betrayed by discovering that a book drew from, say, Hindu or Jewish tradition. I'd just think the same thing I thought when I found out about The Lion..., which was "ah, so he's meant to be X, and he's meant to be Y...." The Lion, the Witch and the Wardrobe is just a wonderful, wonderful book. The moment Lucy realises Mr Tumnus is a spy was the first time a book ever shocked me, and the concept of the other-world-through-a-wardrobe is possibly the greatest idea in children's literature.
Also, once when Tolkien was reading out another bit of his then work-in-progress, The Lord of the Rings, C.S. Lewis turned to a friend and muttered, "oh God, not another fucking elf..." For that alone, I would love him.