I'd say Rihanna. Although I think Christina should be mentioned here too.
Rihanna has done so many genre's and sounds great on them. Regee, Island, Pop, R&B, Rock, Dance, House, Dubstep, Urban, Ballads, Dancehall, etc.
But sounds the same on each one. I'm a fan BTW. She never uses a full rock vocal or a full west indian vocal (on Man down she says 'when me went downtown' -now riri herself should know you'd say downtong....SMH...Gaga does a better west indian singing accent than her)