If you're talking about the real traditional "folk", I'll say Bascom Lamar Lunsford.

If you're talking about folksong revival, I'll say Mike Seeger.

If somewhat more pop and protest/political "folk", Pete Seeger.

In another "folk" arena, which I am not able to characterize to my satisfaction, Bert Lloyd or Ewan McColl.

In what I'll call "classical concert folk", Richard Dyer-Bennet.

You see, you need to sort them out in order to get comparable groups to be preeminent in.

And then there's the horse who doesn't sing.

Dave Oesterreich