Onitsuka Chihiro is the greatest singer of the 21st century, in my opinion. Not just in Japan, but anywhere in the world. She has the skill with rhythms and phrasing of a Frank Sinatra or a Bob Dylan. And their ability to sell the meaning of a song.
And she has a great voice. Or at least she did until her throat operation. I don't know if she has performed since.
A lot of Jpop leaves me cold, but I love her. She grew up listening to American music, and I think her music (but not her lyrics) is more American than Japanese in some way. Her style is something like Sara McLaughlin crossed with Celine Dion. Her lyrics are raw, complex, and poetic.
My favorite song is Memai (Nausea), but other favorites are Ryuseigun (Shooting Stars), Suna no Tate (Shield of Sand), etc., etc.