3 pages in and we've got Katy, Madonna, Rihanna, Beyonce, and Gaga. If someone can figure out a way to work in Britney and Kesha this would be an all out blood bath.
This more than anything says that Katy has a great record label that marketed Katy as a single artist better than any other current pop girl around. However, Capitol has not marketed Katy well enough to sell a substantial amount of albums.
Lady GaGa and certainly Rihanna, however, despite having lower single sales, definitely beat Katy by miles via album sales.
In terms of current (all-around) success, it would go: Lady GaGa, Rihanna, and then Katy Perry.
End of.
Also, just a point, LFN's struggle to reach it to #1 clearly shows to me that Katy's "power" is starting to wane. The pop panel is about at the point where it has overexposed Katy. She'll soon have a single that has a "Alejandro" route. Oh well.
What creates longevity is the ability to CONNECT to your music, not just put something out there that's catchy and look pretty, we all know how that ends....