What about Rina Aiuchi? Her voice is amazingly powerful!! (Too bad she had to retire, but you can always try her music).

Here are some videos:

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PARTY TIME PARTY UP



眠れぬ夜に

I would suggest:

Kana, lots of people listed her, but she is just that good!
Miliyah Kato
Hedemi Uematsu
mini
miray
Jamosa
Hiromi