The SF trilogy beginning with "Out of the Silent Planet" is also excellent for teenagers (or adults).
Lewis's best fantasy is "Till We Have Faces," but it may be a bit too subtle for some readers. It's based on the Greek myth of Psyche and Eros.
I'm also a fan of Lewis's literary criticism and his popular religion books, but that's another story.
I've read the Chronicles of Narnia to five of my children, and when the last is old enough she'll have them read to her, too.
Lewis and Tolkien were both Oxford professors and friends, and the accomplishments of these men have enriched and ennobled us.
I doubt I'll be able to say that about HP