Utada Hikaru is a very good R&B artist, although the R&B label is really only a flavor added to her unique music. A lot of her songs are layered with lush textures of some interesting electronics sounds done tastefully. Although her Exodus fiasco was, in my opinion, way too focused on trying to appeal to her US audience (the song Kremlin Dusk was exceptionally good, though, with even a really cool name), her albums Distance and Deep River are both filled with great compositions.
Ken Hirai is a "truer" R&B artist in that he sticks closer to R&B conventions. If you like R&B, check him out.
Shunichi Miyamoto strikes me as the next Ken Hirai for some odd reason, but his songs are generally more ballads than R&B.
Spitz is a mellow adult alternative rock band.
Noriko Sakai got me into j-music in the first place. She does pop... well... pop.
I've always wondered what Oda Kazumasa is up to now. His voice is reminiscent of Ryuichi Kawamura (ex-Luna Sea), and his music sounds kind of 80's (70's? probably not 60's...) oldies, but more refined. Lots of ballady songs, I think.
Megumi Hayashibara has some catchy tunes in her half and half album. They sound right off an anime OP (if they aren't already).
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There's more, and I haven't even gotten to the actual rock side (right. as if things can be split so evenly. or pop, for that matter...), but I'm tired. I'd give my own personal favorites, but nobody ever seems particularly interested.