Let me say this.. If Britney was actually involved with her music, she'd probably be considered the most experimental. But she clearly doesnt want to do that anymore, which is sad. ITZ and Blackout are two pop gems....
Also Nicki isnt really experimental at all, even though she is one of my favorite artist on this list. As a rapper I dont think she needed to experiment with her sound too much in the beginning because she was KILLING IT with the verses. As PF came upon us, she started to venture more into that pop sound while fusing the rap genre.
Her mistep was PF:RR. There is no real evolution on there. The 'pop' songs arent experimenting at all... So yeah.
As for Gaga.... I'd say Born This Way earns her the right to be called experimental.... But I think we need more albums/songs from her to for sure say that. As of now though, she is... Yes.