Honestly, all of them were pretty experimental except for Katy.
Beyoncé was incredibly experimental sound wise with 4, Rihanna with Rated R and Good Girl Gone Bad, even some of Loud and Talk That Talk, Britney with In the Zone and Blackout, Gaga with Born This Way, even though I despise the album, and Nicki experimented a TON on PF:RR, even if its trash.
Katy is the only one who went from being an experimental type of pop star, to go down the generic route.
Crazy In Love
Work It Out
Baby Boy
Hip-Hop Star
Smash Into You
Sweet Dreams
Irreplaceable
Run The World
End Of Time
Love On Top
Countdown
The Whole IASF album
The Whole 4 Album
The Whole Cadillac Records Soundtrack (Blues) Which I might add none in the poll wouldn't dare do or sound good singing blues.