You can never go wrong with these fantastic singers:
hitomi - Rock, pop, ballads, electro, techno. You name it. She sings anything and she seriously rocks.
Shoko Nakagawa - Pop, rock, ballads, cute! She's freaking adorable and a total dork.
BoA - Pop, ballads, urban, dance. Who doesn't like BoA? Her voice is so strong and powerful. I prefer her Korean songs.
ayaka - Pop, rock, ballads. Newcomer with an incredible voice.
aiko - Pop, rock. Her music is so fun to listen to.
Kaela Kimura - Pop, rock. Like aiko, her music is insanely fun and catchy.
Kou Shibasaki - Pop, ballads and other unique style. She's an actress also. Wonderful vocals.
ERIKA - Rock, electro. She's an actress, but her music is SO good.
Tommy heavenly6 - Rock. She's incredible. A must, along with her band the brilliant green.
Yifei - Pop, ballads. She's Chinese, but she has an incredible Japanese album that you must try. Her voice is so sweet.
Hayami Kishimoto - Pop, up-beat, dance, ballads. Her music is really fun!
Also, I see Erika on your sig. I guess you already listen to Hello! Project then? If not, Maki Goto, Miki Fujimoto and Aya Matsuura are musts!