I agree with you on Amy, Kev. Where the other girls you listed may have decent voices, Amy had that something extra that gave her music that real life, gritty, soulful aspect. She wasn't just singing her tears away in front of a mirror. I'm sure much of it had to do with the Dap-Kings that she recorded with too. They are just amazing and so tight as a band.
Amy could have been the next female singer that put out great recording after great recording. Think of all the oldie but goodies; Etta James, Ella Fitzgerald, Koko Taylor, hell I'll even throw Barbra Streisand in there, etc...etc... I think Amy could have been a singer of that caliber. It's a shame we'll never know what might have been.