Kumi Koda, Ayumi Hamasaki, Namie Amuro
I really really like Aco especially her old stuff from lady soul and nude. There's also Ai and Thelma Aoyama- love their voices :o
Try May'n. And if you're looking for song written by Kajiura Yuki, try FictionJunction since that group solely performs songs written by her.
Ikimonogakari's Yoshioka Kiyoe has such a great voice. One of my favorites, actually. Enough so I bought their first two major label albums: Sakura Saku Machi Monogatari and Life Album.

いきものがかりー「Sakura」


いきものがかりー「花は櫻 君は美し」 LIVE VERSION!
If you like to listen something more like r'n'b, reggae and so on, you should check Lecca. She's amazing and very talented - writes her own songs and lyrics.



Clammbon is a nice group.

Nujabes + Clammbon- Imaginary Folklore
Oh and MINMI is a good solo artist.

Nujabes feat. MINMI- Who's Theme


Nujabes feat. MINMI- Shiki no Uta
Wow, so many new suggestions in this thread! :) Arigatou!

I like Aimer, Ai and Thelma Aoyama the most. Ricky, your suggestions are also pretty good. :)

Another singer I found recently on YouTube is Maki Nomiya. But I've got to say that so far, the songs I love the most come from Kalafina/FictionJunction. :) For example, this: http://www.youtube.com/watch?v=1TV_zKflcUk&feature=share&list=PL-cIahL6Wrw_0bIckbNRBNvJq9nfrRZrH Keiko's voice comes very close to being the exact definition of what I was looking for. <3
She hasn't been mentioned so I'll recommend Maki Chang, since her songs remind me of Adele.



fumika and May J, try this singers :D



i love Mika Nakashima :) her voice is sexy.. ;) i love Crystal Kay, Bonnie Pink, See Saw, Kalafina, Maaya Sakamoto, May\'n, Ayaka, Azu, hitomi, May's, Hitomi Shimatani, Yuna Ito, Namie Amuro, Yuki, many more.. :D and i love Sabao nowadays i mostly love female singers that sing a movie or dorama soundtrack or anime soundtrack.. ^^
I would recommend: Utada Hikaru Ayaka AI Ito Yuna Crystal Kay Aoyama Thelma Kato Miliyah Yui :)