Ayu for sure but I love Utada voice too<3
Chihiro Onitsuka isn't in the list. She has a great voice.
About Mika's voice, I hate her voice :S It sounds horrible in my opinion.

Others: OLIVIA, hands down. Although live, she tends to be a bit dissapointing.

Edit: Oh damn, this poll is old.

Yes, In 2011 she was. Namie has token her place. Girl could deliver, even while dancing like crazy for 2 hours.

Hikaru Utada has the best voice.