Leo Ieiri
Very popular, started off as a miwa/YUI type and moving and changing as time goes by. Writes her own lyrics as far as I know and has a very unique sound and voice.
SHINE
Relax
Digest of her latest live tour TIME
KOKIA
Old school, been active since the '90s, never really been on top of the Oricon chart but she is still touring and releasing regularly. Writes her own lyrics, has a very unique style.
Ai no Melody
Hontou no Oto
sing+
Seriously, she has so much stuff from so many different albums
I can't suggest her highly enough!
Aimer
She's already mentioned but that's how awesome she is!
Kachou Fugetsu
Ref:rain
Anata ni Deawanakereba