I think this one is tough for me. As I find Cyndi's voice and music very unique and appealing. Her song 'True Colors' is one of my favorites by her. I remember watching her on TV performing that song at one of those award shows and was moved by the performance.
Cher would be second as I like her unigue voice. her songs 'Cherokee woman' and 'Believe' are a couple that come to mind that I find a strong story line in and show her skills. That is my long answer.
Short answer: Cyndi Lauper then second Cher
Madonna will be my vote for the most outstanding singer! She has a wide vocal range unlike all the others. Listen to the way she sings "Open Your Heart" and "Don't Cry for me Argentina." My second choice will be Janet Jackson. Although I must admit, she only sings in one range all the time but her voice is very soothing.