You listed Britney earlier. Do you see Femme Fatale as being experimental?
Some what. Including dubstep influences into pop songs. Blackout, and also In The Zone were extremely experimental--especially during those times. The "Britney" album is somewhat experimental as well with songs like "Slave 4 U", Boys, etc.
Also
singing the same style over different styled beats =/= being experimental
it's not just about production, it can be lyrics, vocal, etc
So Gaga might be mainly [genre]pop but that doesn't mean she hasn't experimented with her voice, lyrics, etc in various, experimental ways in her songs