Two weeks ago, we took the kids to the Mississippi Symphony (yes, we have one of those), and in his introductory remarks conductor Crafton Beck called the Adagietto of Mahler's Fifth Symphony "one of the most beautiful pieces in all of classical music."
I listened to it then and again now. It is absolutely true.
If you are a stranger to classical music and are looking for a transformative experience, find a recording of this piece and listen to it a couple of times. At 12 minutes long, and slightly off the Mozart-Beethoven beaten path, it is a beautiful place go to explore one of the truly transcendent places classical music can take you.