Probably To Kill a Mockingbird. Loved it when I was about 15 and have re-read it a number of times since and still love it. One of those almost timeless, classic books. It has so many lessons to teach about love, loyalty, family, racism and bigotry to name a few.
I also loved this book and enjoyed the film. Gregory Peck won the Academy Award for his portrayal of Atticus Finch and I can never re-read the book without seeing this lovely actor in this "role".