I wonder if we are talking about innovation or popular practitioners?
I would have to put Doc Watson in the category of a definitive folk singer as much as I would Pete. Has bluegrass left the folk music category or is it as Alan Lomax said, "folk music with overdrive"? Then Bill Monroe or Earl Scruggs pops up. What about Jean Ritchie?
(Talking American folk music here.) The Kingston Trio had far reaching influence on the "folk scare". What about Burl Ives? Wouldn't he be influential? A case could be made for Woody, Leadbelly and even Josh White, who brought blues to the supper club.

Is Pete Seeger truly representive? He plays ninths and thirteenth chords in Appalachian music. Mike doesn't. He plays southern music like a northerner. Nothing wrong with that but what does that define?
Regional music? He also plays a unique sophisticated style of 5 string banjo which he innovated. No traditional player from the south plays like that. See the problems that this all engenders?

What about African-American folk music? More of the same.

Frank Hamilton