I'm gonna throw this one out there... B'z (and Porno Graffiti)!
If you want some non-idol girl groups, there's SCANDAL, FLiP, Rin', Stereopony, NEGOTO, and Silent Siren
(their sound is a little sweet).
Solo artists, I would recommend YU-A, BENI, Crystal Kay, Aya Kamiki, Ms. OOJA, ALi, Meisa Kuroki
(I'm not sure if you listen to her or not...), JUNIEL, Eir Aoi, Nanase Aikawa, Marie Ueda, Kou Shibasaki, Leo Ieiri...