I think Cher played a part but Madonna set the standard. I believe if it hadn't been for Madonna there wouldn't be a Britney, Christina, Rihanna, Lady Gaga, or Katy Perry. Now whether they are gifts are curses, I think it says something about Madonna's influence on pop music and pop culture. So Madonna wins to me.
Whitney and Mariah are important but I agree with philli, they helped shape R&B music more. Besides, when I think pop artist- I think complete package.