Adele is the biggest right now, Rihanna and Katy/Taylor would come after them.

Overall is:

Madonna
Céline
Mariah
Whitney
Britney/Rihanna/Gaga

Currently it's Katy, Taylor and Adele. Their last eras were much, much bigger than any other female artist's.

Madonna, Celine and Mariah will remain the trinity, though.

Rihanna is

Adele and Taylor/Katy

there are more than 3

Rihanna, Adele, and Katy Perry.

Adele, Katy and Rihanna right now

It used to be Gaga Idk tho

My main faves are: Katy, Rihanna, and Adele

Shakira, Madonna, Rihanna

I would say Shakira, Beyoncé & Rihanna...since, as someone mentioned earlier, Madonna is pretty much in a league of her own.