My choices would be based on the ability to sing on Key, to able to carry the notes and to understand, musically, what you are meant to be conveying..
I will leave out the opera singers since all seem able to accomplish these things...
June Tabor, Ella Fitzgerald, Jo Stafford, Louis Armstrong, Al Bowley, Roy Orbison,Amy Winehouse, Anne Murray, Edith Piaf, Norma Waterson, Aretha Franklin, Sarah Brightman, Judy Garland . Lot more but these come instantly to mind. A very interesting thread, thanks.