Kuu for me.
Even if she did overdo some lives, she's mostly on pitch, has a great range and much emotions and power in her voice (Moon Crying at Taiwan live, just pure love

).
Namie would be second for me, cause it's pretty and comfortable to listen to in her last tour. But it's really weak sometimes and even if she can greatly dance and sing at the same time, you aren't able to tell sometimes who's the actual singer.
And Ayu... *sigh
I really think Ayu can't sing properly anymore. I love her live voice from 2003-2006 though.