Hard to place them in any order and like any list it can change day to day, or minute to minute...
Also Im not too fussed if a singer actually writes their material, it plays a small factor but I go mainly with their voice and how they own a track...
Whitney (crazy powerful voice, great control and can bounce on a track like she did with 'its not right/but its ok)
Mariah (she used to hit high notes like no other)
Aretha Franklin (the queen of soul)
Sade (that hoooot voice over soulful jazzy beats is so unique)
Bettye Lavette (mad versatile, not even sure what genre she is r&b, soul or blues???)
Erykah Badu (part of the OG neo soul movement, distinctive voice, has subject matter in her songs and also a great live singer too)
Dusty Springfield (more of a folksy/soul/bluesy singer, but one hell of a voice and like Anita Baker can make you really feel a song)
Anita Baker (one of my favourite artists when it comes romantic ballads, she brings mad emotion to a track)
Lauryn Hill (loved her song content and her style and also versatile and a great singer and my goat female rapper)
Minnie Riperton (also great with high notes but made one of the most beautifulest songs in the world whilst dying with cancer)
edit: just tried to placed them in an order best i could..