Britney over Rihanna?
Her first 3 albums had the exact same sound... Especially the first 2.
In The Zone is flawless... So is Blackout... Both were new sounds for Britney... Back to Circus.. Not at all. Femme Fatale... Hardly at all.
Rihanna on the other hand, changes her sound CONSTANTLY. She may not have done every genre under the sun, but she is incredibly versatile and has pulled off the most genres the best. So there.
Beyonce also experments, but I wouldnt say to the extent that Rihanna has... 4 is a masterpiece, but her other 3 albums didnt stray too far from her R&B/Pop roots.