I think Deskjet has the major part of the answer: a genius has to be a revolutionary, to change the existing order. The term is way overused; in physics Newton would certainly head any list, but I'm not sure I'd put anybody from the 18th century on. . .in the 19th century you have Maxwell for sure. . .
   in music, Van Ronk had Bach and Ellington on his list. Like others, I'd add Mozart and Beethoven.
   In folk music, our reverence for tradition is inconsistent with genius. Genius involves creation, and good traditional music IMHO means that the area in which creation is allowed is so limited that the level of creation can't approach genius. Hazel Dickens both wrote and sang "It's hard to tell the singer from the song." She was right, and it precludes genius.