It had been a few years since I read the Narnia series, but, if I had had to pick a favorite, without re-reading, I would have named The Silver Chair. I had forgotten some of the reasons. Most of them are set out below. They involve a really great character, Puddleglum, the quintessential pessimist, and many moral choices/temptations.
In The Silver Chair, the Pevensie children do not appear. Eustace, their cousin, from The Voyage of the Dawn Treader, who was a "record stinker," and also became a dragon, but was cured of both of these, is involved. He involves Jill Pole, a schoolmate. The two of them are drawn into Narnia, as they are trying to get there to escape hazing from schoolmates. Aslan has called them to Narnia to find the lost prince, Rilian, King Caspian's son. (This is the same Caspian as in the previous two books, Prince Caspian: The Return to Narnia, and The Voyage of the Dawn Treader. He is much older in Silver Chair. Eustace is a little older than he was on his first trip to Narnia.) They come into the same planet (?) as Narnia, but enter it on a high mountain, overlooking Narnia proper, a long way from it. This appears to be one part of Aslan's country.
Jill is tempted to not drink, and not to trust the Lion, but does trust him.
Jill and Eustace are tempted to squabble over Jill's actions on the mountain, and do.
Caspian is tempted to kill Drinian, who did not stop Rilian from seeking the green lady. This would have been an example of wrath, one of the seven deadly sins. He does not do so, and forgives Drinian.
Rilian was ensnared by the green witch. Was this a temptation? If so, it was probably partly lust, one of the seven deadly sins.
Jill is tempted not to repeat the signs Aslan has told her to remember and repeat to herself. She succumbs:
. . . whatever the Lady had intended by telling them about Harfang, the actual effect on the children was a bad one. They could think about nothing but beds and baths and hot meals and how lovely it would be to get indoors. They never talked about Aslan, or even about the lost prince, now. And Jill gave up her habit of repeating the signs over to herself every night and morning. She said to herself, at first, that she was too tired, but she soon forgot all about it. And though you might have expected that the idea of having a good time at Harfang would have made them more cheerful, it really made them more sorry for themselves and more grumpy and snappy with each other and with Puddleglum. C. S. Lewis, The Silver Chair, New York: Macmillan, 1953, p. 77-78.
This isn't any of the deadly sins, unless it's sloth. My take on this is that apathy and neglect are as bad or worse than any of the deadly sins.
Jill and Eustace are tempted to pay attention to the charm and beauty of the green witch, and do so.
After missing two of the signs, and also beginning to eat talking stag without realizing it, Eustace, Jill and Puddleglum escape the giants of Harfang house, and get into a vast system of caves. They are tempted to doubt the authenticity of the sign that they followed in getting into the subterranean realm.
They saw the ruined city of giants out of a window in Harfang, and saw that UNDER ME was carved into the pavement. Jill had been told to look for a writing on the stones in the ruined city. They are captured, and, finally, brought to Prince Rilian. (He is enchanted, and doesn't realize who he is, except for an hour every night.) He mocks their interpretation of the meaning of this writing. Puddleglum responds:
"Don't you mind him," said Puddleglum. "There are no accidents. Our guide is Aslan; and he was there when the giant king caused the letters to be cut, and he knew already all things that would come of them; including this." C. S. Lewis, The Silver Chair, New York: Macmillan, 1953, p. 131.
They are tempted not to follow the fourth (and last) sign, when Rilian, sane, but tied in the silver chair that renews his enchantment every night, asks them, "by Aslan himself" (p. 141) to cut him loose. They decide to cut him loose.
They are tempted to believe the witch, who tries to persuade them that there is no world aboveground, no Aslan, and enchant them all, as well as enchanting Rilian again. All try to resist, but are almost ensnared. Finally, Puddleglum extinguishes the witch's fire, which was producing enchanting smoke, by stamping on it with his bare webbed foot, and frees them from the spell. This illustrates I Corinthians 10:13, where God promises a way of escape, if we want one.
Aslan forgives them for the temptations succumbed to:
They turned and saw the Lion himself, so bright and real and strong that everything else began to look pale and shadowy compared with him. And in less time than it takes to breathe Jill forgot about the dead King of Narnia and remembered only how she had made Eustace fall over the cliff, and how she had helped to muff nearly all the signs, and about all the snappings and quarrellings. And she wanted to say "I'm sorry" but she could not speak. Then the Lion drew them towards him with his eyes, and bent down and touched their pale faces with his tongue, and said:
"Think of that no more. I will not always be scolding. You have done the work for which I sent you into Narnia." C. S. Lewis, The Silver Chair, New York: Macmillan, 1953, p. 202.
And there will be an end to temptations:
"Sir," said Caspian. "I've always wanted to have just one glimpse of their world. Is that wrong?"
"You cannot want wrong things any more now, that you have died, my son," said Aslan. C. S. Lewis, The Silver Chair, New York: Macmillan, 1953, p. 205.
I have included another quote from this book in a post on a different subject.
This is the fourth in a series, which began with this post.