How can anyone normal put Katy Perry above Beyonce,Lady Gaga and Britney??
She may have her #1 songs and successful album at the moment, but she ill never be as big as them. No one takes her seriously.
So:
A- Madonna,Lady Gaga,Beyonce,Rihanna,Britney,Adele- people who actually have(had)influence on pop music and culture and changed things .
B- Katy Perry, Taylor Swift,Miley Cyrus,Pink - girls who are really great at being a pop star, but that's all they are. Only pop stars who follow trends and play it safe.
C - Lana Del Rey,Kesha,Jessie J,Nicki Minaj - those girls get people very excited, but the excitement dies pretty early also.
D - Selena Gomez,Ariana Grande,Demi Lovato,Lorde,Iggy - they need more time to prove that they can be really great pop stars in a future.
This is about overall success.