TAE YEON from Girls' Generation!!! She's Korean though. But she's wonderful. She's like... 20 years old? She voice is so husky and her vibrato sounds amazing. Unlike Ayu's, haha.
If it's only for Japanese vocalist, then it would be
ayaka and BoA for me!
Ayu doesn't sing so well live. Her voice in general can be good or really bad. She used to be amazing around the RAINBOW era. Now it's just bad. Really bad. She can't hit her high notes and her vibrato are terrible. XD
I don't like the voices of
Utada and Mika so much... But Mika can hit her high notes. Props to her.
Koda Kumi is great in performance and she often sounds exactly like her CD version but her voice in general is scratchy, not very pleasant.
Namie Amuro is wonderful on stage also. Her range is very limited but beautiful voice.