Totally agree with those that nominate Pete Seeger, who I believe had the most profound effect on defining, interpreting and spreading folk music...

Don and Mick spoke of Joe Hill, who saw music as a means to simplify a message so that language and cultural barriers could be bridged, and people could identify their common interests...

I'll add a couple others, Merle Travis/ Chet Atkins, and Uncle Dave Macon, for their effects on the traditional music that became "country".

And, although I don't know that they played anything more than the field recorder, John and Alan Lomax sure had a major influence in preserving this music.