I think that's up for debate. You could make both arguments: a) Britney's debut overshadowed Christina and destined her to be forever the runner-up pop star b) Britney's debut ignited a hunger for young blonde pop tarts and elevated Christina even more
Born this way 7M album 180M tour
Teenage dream 6 with a re-release 60M tour
gaga win
In no way the BTW era was bigger than TD other than touring. The massive amount of singles Katy sold more than makes up for the million more albums (400,000 of them 99 cents on Amazon) Gaga sold.
Beyonce if Rihanna didn't exist. Come on, during the Britney VS Xtina days Beyonce was in the shadow of both of them. It wasn't till modern days Beyonce wanted to be the #1 but she always getting cockblocked by Taylor, Rihanna, and Britney