Woody Guthrie died in 1967. Many of the nominees reportedly made pilgrimages to see him, even though he was way beyond being able to sing write or perform, I think he was still the most important folksinger of the sixties.
I think Pete Seeger followed close behind, because he had been blacklisted for years. Nothing proves your effectiveness more than when your enemy attacks you.
Then in no particular order, Arlo, Baez, Dylan,(pre-electric), Ochs, Havens, Odetta, Paxton, followed by the commercially successfull, PPM, electric Dylan, et al.