While it is difficult to pick between the final three books in this series, I think that The Silver Chair is my favorite book of the seven. I love the relationship between the two kids (Eustace from Voyage and Jill Pole, a picked-on girl from the Experimental House school in England), the marsh-wiggle Puddleglum, who, as his name suggests, has difficulty in seeing the bright side to life, and the trapped Prince Rilian. These three aspects makes The Silver Chair an exciting story.
In my review of Voyage of the Dawn Treader, I mentioned how the undragoning scene was one of my favorites in the series. Another is in this book regarding the enchanted silver chair. The kidnapped Prince Rilian is under the charms of the evil witch, but has an hour each evening in which he remembers Narnia and Aslan. He is tied up and bound to the chair until his fit passes. The enchanted Rilian acknowledges his fits, and agrees to be tied to avoid these nightmarish thoughts. However, we see that that hour is his true self coming through, and the rest of his life he is under the spell, not the other way around. The application to our lives is clear: we sometimes forget that this is not the real world, but mere Shadowlands, and that we belong to a much better kingdom and follow a more powerful master. We allow the world and its objects to bewitch us and cause us to forget our true home. The key scene in this book is when Eustace, Jill, Rilian, and Puddleglum are attempting to fight the witch, but her soothing voice and plucking harp lures them away from their mission. I hope some pastor out there uses this as a powerful sermon illustration!