In an unrealistic world -- if they continued their careers on a current path, maybe Demi for Pop Vocals? Unrealistic world with different paths -- if Miley went completely Country as she got older.
Demi if she was a little more respected.
Miley if she straightens up.
Selena I love you sis, but if you stay under HR, you aren't getting any type of Grammy.
Miley, because has a nice voice and is the biggest of the 3.
Demi is good enough for one, but she is nowhere near as popular as Miley.
Selena won't get one