I see alot of people mention Namies voice to be better, because she can dance too? So that makes her voice better? Put the dancing and scene performance at side, and think about voice only, then namies is still good, but not the best.
And also, just because someone can't really dance, should not judge their voice on it, I never see Utada dance, but then again, her style and personality is not so dance-y, and if she started to dance it would look silly. So it's better that she doesnt. It all depends on the artists and his/her style and image really.