Who believes that Robert Plant

Best woman, Linda Ronstadt.
Country, rock, opera, pop, torch music, etc.
She could do it all.

Now back to Robert Plant.........

Joan Jett and the singer of The Breeders. For male singers Kurt Cobain isn't exactly chopped liver.

Plant. There must be a place near the top for Stevie Winwood.

As far as women singers go, you would need to mention the only woman to sing on a Led Zeppelin album with Robert Plant, Sandy Denny. She was a seminal figure in the British folk music scene.