Impossible question so will confine myself to female singers I've seen live...

The incomparable Sandy Denny. Just listen to "Banks of the Nile" on "Fotheringay".

June Tabor - I will never forget her singing "The Band Played Waltzing Matilda" at the Enterprise in N. London. A very hot Sunday evening, all the windows open, red buses stopping outside every 5 minutes. In spite of all that June's singing and Eric's song moved everybody to tears, and she had tears running down her face too.

Maybe a suprise, but Linda Thompson. Saw her do "Pavanne" once & she nearly frightened me to death!