I realize that the question asked about the most influential folk musician, but I don't really like distinctions in music.

I believe the artists who have had the most influence on the music of today are Robert Johnson, Muddy Waters, Chuck Berry and Bob Dylan. Yes, they had mentors, but I think Johnson was the finest poet of his genre as well as having honed what he learned from the guitarists of his era, Patton, House etc into a style that became the basis for the Blues.

Muddy Waters and his band (I sure don't want to down play the contribution of Little Walter and Willie Dixon, but it was Muddy's band) defined the Chicago sound that in my opinion was the direct father of Rock and Roll.

Chuck Berry took it to the next step, and say what you like about Lonnie Donegan, The Beatles and Stones were hugely influenced by the sound Chuck Berry created.

Bob taught us that it was okay to say something other than "tell your ma, tell your pa, our love's gonna grow, oowha oohwa".

I guess that George Martin and Phil Spector should also be mentioned.