Jazz is essentially a local American music; folk on the other hand is a collection of local musics from all over the world, so it's just impossible to have an analogous defining individual.

For America Pete Seeger seems a good choice, because he links in to so many other things over such a large chunk of the century, including the Weavers course, but also including Woody Guthrie, Bob Dylan, Steel Band music...

For England very likely Martin Carthy, or better the whole family. For Ireland it's hard, but perhaps Christy Moore, though it really should be Sean O Riada.

And so on and so forth.