Dionne Warrick, Anita Baker, Donna Summer, Melba Moore, Janis Joplin, Barbara Streisand.
Some had more notoriety and more of an audience depending on decade and songbook. For my money - that's the group I'd say were all on a par.
There are so many great vocalists but for the late 60's and 70's Aretha and Streisand held that throne.
I was always a big Ella fan.