Are we talking about currently? Britneys latest era flopped but that doesn't erase all the huge accomplishments she's achieved in her 15 year career. I think it's:
Britney>Beyonce>>Taylor>Katy>Rihanna
It's strange how Taylor isn't that big in Europe. I don't get it
She is doing fine in my country (in Europe).
Anyway its easily Taylor. Her name is known all over the country, biggest female digital song week (623k), effortless multiplatinum albums for 8 years in a row, tours grossing near 100 mil from there alone.... etc.
She isn't even "that big". Most of people I know knows a single Taylor Swift song I only know IKYWT and it's just because of ATRL
I know all her singles because I love her but her first real hit in France was We Are Never Getting Back Together And it wasn't even only thanks to her, but thanks to a parody
Your list must be reversed. 
But still voted for Katy 
