Aretha, without a doubt. She broke down sales barriers so other black women could sell and ran laps around the other black females of her time. Don't get me started on her influence and legendary status. American Idol made their contestants sing around a video screen of her because she didn't want to fly out to them
I would say either Whitney or Mariah is next (I personally prefer Mariah, but it may be Whitney due to her impact), followed by Janet, followed by Beyonce.