you should have included something from Britney's first album, like From The Bottom of My Broken Heart. It really shows a different side of her vocals.
I would say Rihanna's voice is the most amenable to different sounds. I would say Britney right after her. Gaga and Beyonce's voices are really strong, dark and deep which I think may hurt them when they need to use lighter vocals although they excel in genre's where that dramatic tone and ability is valued and needed.
Beyonce, then Rihanna. Both of them have highly adaptable and versatile voices.
Part of the reason I have to give it to Beyonce overall, though, is because she's able to sing songs like Smash Into You with poise and clarity, then turn around and rough it up with songs like You Oughta Know, perfectly emulating the vocal style of Alanis Morissette herself.