Impossible for me to pick one in any genre let alone overall. Johnny Cash, BB King, Bob Dylan, Ray Charles, Paul Simon, Jimi Hendrix, Jerry Garcia, Stevie Wonder, Bob Marley, James Brown, George Clinton, David Bowie, Michael Jackson, and Prince are all sort of equal pillars at the height of influential solo artistry to me. Then a whole different list of bands.
I love MJ too but I have trouble picking just one favorite (a problem I have in a few areas.). I think because each one brings me enjoyment in different area. For example, Andrea Bocelli and MJ, two very different artists but both touch me in very different ways.