Which Narnia book is best?

David Mills at Touchstone's Mere Comments starts off a discussion of which Narnia books are favored by his readers:

Today our youngest and I read almost all of The Last Battle, the last of the Narnia Chronicles. It is my favorite of the set, perhaps because it so well mixes melancholy and hope, or better, transcends melancholy with hope. I find the last chapter almost unbearably moving.

So today's question is: which of the set is your favorite, and for that matter which is your least favorite, and why, if you care to say. (For some of you "least favorite" may mean in some cases "one you most dislike.") My least favorite is The Lion, the Witch and the Wardrobe, because it is the most didactic of the series and the analogies too straightforward and obvious. (read the responses and join the discussion.)

Touchstone Magazine - Mere Comments: Your Favorite Chronicle