I think putting Britney, Beyonce, and Christina is kind of pointless...the new girls have very much copied most of what they used to do back in the day. And also, one of those girls does the copying more than being copied
I don't know.
Nicki Minaj, Pixie Lott, Selena Gomez, Jessie J all scream Katy Perry often.
You have several artist following Rihanna too, i.e. Rita Ora
But Britney probably the most.