I get what you all are saying about Gaga but in my opinion Rihanna has been the most experimental when it comes to her albums.
Pon De Replay
Dont Stop The Music
Umbrella
Disturbia
Firebomb
Mad House/ Red Lipstick
California King Bed
Man Down
Cockiness
Rockstar101/G4L
Te Amo
The list could go on and on. Nearly every single Rihanna album has had a different sound/ direction with different influences. Gaga has obviously experimented with different sounds using her voice in different ways also but Rihanna and Britney seems to be the ones that has done it all in my opinion.