The Uglies Series - awesome dystopia/futuristic, good messages
Ender's Game - Love it, named my cat Ender after the main character (mc reference is just a bonus) They made it into a pretty good movie too. I'm reading the sequel now, Speaker for the Dead and it's very good
Chronicles of Narnia by C.S. Lewis - look for spiritual messages, some very good theology in there
The Hobbit and Lord of the Rings - Tolkien wasn't a Christian (at least at the time) but he was searching for God. Some good spiritual messages. He was buddies with C.S. Lewis.
I also like the Percy Jackson books, both series. It is Greek/Roman mythology
Oliver Twist, A Connecticut Yankee in King Arthur's Court, Treasure Island, Robinson Crusoe, Swiss Family Robinson (abridged versions)
Spiritual Most of these a teen could read, great content without fancy theology words.
Velvet Elvis by Rob Bell - opens your mind, talks about what it means to be Christian and what things matter and what things don't
Blue Like Jazz by Donald Miller - more narrative style, also about Christianity
Crazy Love by Francis Chan - awesome book about God's love
The Great Divorce by C.S. Lewis - Allegorical, amazing book about heaven and hell.
Wholeness in Christ by William M. Greathouse - Fantastic book about Holiness.
Mere Christianity by C.S. Lewis - Difficult read, skip the first section if you're having trouble, the last 2 are much easier. Also about Christianity in general
Redeeming Love by Francine Rivers - Retelling of the story of Hosea, reads like a romance novel but real love
Captivating - Book about womanhood, I don't agree with everything but it's pretty good
The Shack - allegorial, somewhat controversial, about the Trinity and character of God, reads like a novel, I don't agree with everything but highly recommend it
Sex God by Rob Bell- He says he's talking about sexuality (broadening our understanding) but he's really talking about humanity. Excellent book on what it means to be human and the restoration of our humanity (the image of God in us) (side note: we shouldn't say, "I'm only human. To be truly human is to bear the image of God, humanity is a beautiful thing that has been corrupted by sin and will be restored, not destroyed, in the end. When we say "human nature" we most often mean "sinful nature")