Oh, this one's easy. It's got to be Peter, Paul and Mary. They are probably the first folk group I ever heard. Their songs, Blowin' in the Wind (I know, written by Dylan, but Peter, Paul and Mary's is the version I love best), Puff the Magic Dragon (which they say was Not about drugs), If I Had a Hammer, Leavin' on a Jet Plane (written by John Denver, but immortalized by Peter, Paul and Mary). They were so much a part of the 60s and consequently, of my growing up years. I got to see them a few years ago, about a year before Mary Travers died. I am so grateful for them individually, for their music, for their exquisite harmonies, and for what they stood for and how they helped those of us coming into our own, understand certain values and how to stand up for them.
Do you know their music? Here's a link to Blowin' in the Wind on YouTube: http://www.youtube.com/watch?v=3t4g_1VoGw4