It's obviously Katy.. anyone voting otherwise is either not a fan or is a bit delusional.
(#4) Rihanna-Loud
3 #1 hits and CKB/MD still going, good sales.
(#1) Katy Perry- Teenage Dream
Good sales, 4 #1 smash hits and another #1 coming.

Breaking radio records and chart records left & right.
(#5) Britney Spears-Femme Fatale
Decent sales, 1 #1 hit and a moderate hit.
(#2) Lady Gaga- Born This Way
Amazing first week sales, 1 #1 smash hit so far with 2 good-selling non-singles, 1 top 10 promo single, 1 moderate hit, and 1 hit coming towards the #1 spot, breaking chart records left & right.
(#3) Adele- 21
Amazing sales, 1 #1 hit so far & era is only on first single (in U.S.), breaking chart records left & right.