you know, not to be a downer on your question but I don't think I could pick just one song. Literally I couldn't. This is because I can't pick one song that sums up what they are, one song that explains something in my life, one song that makes me happy when I hear it, one song that I would never get tired of, because there is no one song for me. The Beatles are my song!
but since you did ask so nicely I would have to say...Help!