I have read most of the thread and will only nominate names that appear infrequently.
Stacey Kent
Kari Bremnes(Norwegian folk singer)
Kate Rusby(English Folk singer)
Sara K
Eliza Gilkyson
Sarah Massen (particularly, Dreamlife of Angels)
Lori Mckenna
Rosie Thomas
Shawn Colvin
Opera:
Maria Callas of course, best choice Tosca
Vittoria De Losangeles Beechams version of La Boheme, one of the great opera recordings, her entrance in Act 1, is sublime
Elizabeth Sharzkopf, Strauss Der Rosenkavalier, another of the great opera recordings of all time
Janet Baker, In anything, burt Cavali's "Callisto" is a good start