The Christy Miller series is probably my favourite Christian teen collection. The writing is really good, the characters believable, and indentifiable. I really learnt a lot about family and God from reading these books, and I hope that plenty more people will read them and be changed by them like I was.
The Katie and Sierra series are also incredible, and I would strongly recommend reading other books written by Robin Jones Gunn, such as Glenbrooke and the Sisterchicks.