Why do you hate HIGH and MIGHTY COLOR? T_____T My favorite band.
Anyhow, if you like the mellow rock genre, try REMIOROMEN. I also like Cocco, fra-foa, SHAKALABBITS, Tommy heavenly6... Maybe JUDY AND MARY too. But the front singer's voice can get a little annoying at times.
I'll keep in touch with this thread for the J-Rock suggestions! I haven't really found a lot of artists I really like... I can't find more bands like HIGH and MIGHTY COLOR! Or at least Tommy heavenly6.
As for J-Pop: BoA, aiko, the brilliant green, Namie Amuro...
I highly recommend hitomi. She's THE best.