Really hard to narrow it down to one name. Certainly strong arguments could be made for Pete Seeger, Woody Guthrie, Burl Ives (ask your parents, and their parents)or F J Child.

I'd put my vote in for Alan Lomax, however, for several reasons: Sheer longevity, songs collected (with his dad) that are now part of the repertoire, international influence, (he lived in Britain for a few years and was credited by MacColl as a major influence) introduction to the (folk) masses of several major artists, and sheer force of personality. I never met him, but EVERYONE I've talked to who has, was affected by him.