I want to say Rihanna, but Britney's discography and impact on pop culture stomps all over her.
Also, You never know what Beyonce will do. She could do more I am Sasha Fierce albums, B-Day or more 4 albums.
I feel like 4 was mainly for her fans, while "I am" was a little bit leaning in the opposite direction.
It really depends on what she wants to do and does.