I think that we'd have to wait until long after they're dead to determine their place in the musical hierarchy, just as we have to do for artists in other forms of art.

JS Bach towers over his children (except for PDQ, of course). But whether Johnson surpasses Coltrane is up for discussion, as is the superiority of, oh, Woody Guthrie over Pete Seeger or Joan Miro over Pablo Picasso or Shakespeare over Lope de Vega.