A brief review of the series, which I read this summer. The books are of varying quality, and though they are beloved, and particularly good as children's books, ultimately I don't think they achieve the quality level of The Lord of the Rings. Book by book, in order of date of publication.
The Lion, the Witch, and the Wardrobe: If you are going to only read one book from the series, this is the one to read. It combines clever escapism, moralism, and hardly-subtle-at-all Christ imagery involving Aslan. To think: when I first read this as a child, the connection between Aslan and Jesus completely escaped me! Amazing! The good aspects of the book: Lucy the curious girl, Edmund the selfish boy, the Witch, and Aslan. The older children are undeveloped in comparison, and the creatures of Narnia are pretty uniform. For me, the aspect of the book I dislike the most is how these four children are suddenly the kings and queens of Narnia. And we thought 'a moistened bink lobbing a scimitar' was a poor way to decide upon supreme executive power! If you are intent on reading the stories in chronological order, the prequel should be read first. But I recommend reading this book first.
Prince Caspian: This is one of the weaker books. It reads like a sequel, with not a heck of a lot of thought given to developing the world more. It's a few years later in England, but centuries later in Narnia, and the kids are needed to put things right, including putting in Prince Caspian on the throne. Lucy is again the beloved child, and Susan is starting to become less reliable (a plot development that seems to be completely unnecessary). The ending is an exercise in deus ex machina, but without the religious lessons of the first book.
The Voyage of the Dawn Treader: possibly the best of the books, certainly the best of the four featuring the English kids. (I'm not counting The Last Battle.) This books decides that Peter and Susan are too boring to bring back, so they are replaced with Eustace Scrubb, who appears to be a child of freethinkers or modernists or something, in any case the kind of person who earns C.S. Lewis's scorn. :) The general story line is that Lucy, Edmund, and Lucy are on the Dawn Treader with Caspian and Reepicheep, and they have adventures sailing to the edge of the world. There are a number of different stops along the way, and they provide for interesting separate episodes. Lucy has her final interesting adventure in this book, and the evolution of Eustace is pretty good. All in all, I'd say Eustace is the series' most interesting character. The end of the story introduces the edge of the world and Aslan's country, which will become more important in later books as Lewis fleshes out his ideas of Narnia's structure. But the better parts of the book are simply the adventures.
The Silver Chair: No more Lucy and Edmund, but we still have Eustace and a girl who goes to the same school, Jill Pole. Jill really never gets developed well, either in this book or in The Last Battle. The two kids are set on a quest by Aslan to find the Prince and heir to Caspian's throne. Aslan spells out exactly what they should do, but they still manage to mess up his instructions. They go North with the 'Marshwiggle' Puddleglum, have adventures with giants and eventually find the prince underground in a cave network run by a witch who is suspiciously like the White Witch of LWW. The adventures are decent, but then the book gets into examining dilemmas of considering belief and reality. It doesn't do well with them. It reads like a diatribe against atheism, which would be one thing, but it goes to the extreme of saying 'well, we don't care if our beliefs are true or not - since they make us happy we'll stick with them'. This little bit nearly ruins the whole book.
The Horse and His Boy:
Instead of recycling yet another story about English children in Narnia, like the first four books, this one concerns a boy and a horse native to Narnia. (This raises the side question: why are there no humans in Narnia at the beginning of the series, but starting with the third they start being quite common and, by the last three books, they are all over the place?) The story is pretty good, and works well as a standalone book, even though Edmund and Susan make an appearance to tie the book into Narnia. Still, the series is starting to get a bit creaky.
The Magician's Nephew:
Apparently C.S. Lewis was getting tired of Narnia so he decided to wrap of the series with a creation myth and an end-of-the-world story. The creation myth is this book, and it's far better than The Last Battle. As with most of the books, the story follows the path of children, in this case a boy and a girl named Digory and Polly. Digory's uncle has made magic rings that allow one to travel between worlds, and this leads to the discovery of a number of different worlds, and eventually the discovery of Narnia as it was being created by Aslan. The manner of inter-world travel was quite clever, and the introduction of the woman who was to become the White Witch is pretty well done. On the whole, one of my favorites of the series.
The Last Battle:
I'm trying to decide why I dislike this book so much. First off, there are two parts of the story: a conflict involving an ape and a donkey undermining faith in Aslan, that ends up leading to the invasion of Narnia by foreign forces. But then the story takes a turn into a world destruction myth that appears suddenly and without much need or sense of being there. What's the moral to be learned? Hard to tell. As a story, the first half is tolerably good. It's a reasonable investigation of what happens when people use religion to bamboozle. The ending is just uninteresting. It seems to be an explication of Lewis's theology of Shadowlands. The world we inhabit is merely a shadow of true reality, and after we leave this one, if we have the spiritual fortitude to hold onto our faith, we start climbing into greater and greater realms of reality. So the last fifty pages or so consist of Lewis waving a wand and creating his own heaven. 'Everything is increasingly wonderful' - it is a simple theme and not of Lewis's better ones.
In summary, I would put MN and VDT at the top, with HB and LWW, followed by SC and PC, with LB the least interesting or compelling.