I think one of the messages that has always stood out to me has been the dwarfs in The Last Battle. While their stubborn refusal to see what is in front of them has its roots in their unwillingness to submit to the authority of Aslan, a big part of their blindness also resides in the bad view/experience they had of [the fake] Aslan and his followers. It always reminds me that, as a Christian, I need to be living the way God wants me to be living every second of the day, both for Him and for those I don't know may be watching my example. Because just saying I'm a Christian doesn't necessarily mean I am a Christian, it's also in the way I act. How horrible it would be to turn someone off Christ and His eternal plan for them! So yes, that has to be one of my favourite pieces of imagery.
Another is at the end of Voyage of the Dawn Treader, where Aslan says, "I am also in your world, but I there I go by a different name. You must learn to know me by that name." I LOVE that. It's so clear, and it manages to make my heart beat with excitement every time I read it!
Lastly, I think the last paragraph of The Last Battle is perhaps the most beautiful piece of literature I have ever read. When it talks about how
Sorry, I know it was supposed to be favourite message, but I really couldn't choose!