The best in each category of japanese music:

JPop: Ayumi Hamasaki (she is a godess, all her music is wonderful and means a lot to me)
JRock: DespairsRay (some would categorize them as metal, but they are mainly rock with a heavy edge... great band)
JMetal: X-Japan (one of the best bands ever,heavy fucking metal to the core)
JRap: All japanese rap sucks.

Other good acts which I currently listen quite a bit to: Nami Tamaki (jpop), See-Saw (jpop), Dir en grey (distorted jrock), TM Revolution (jpop with a techno/eurobeat flavor).

That should keep you satisfied.
And yes, I know that the topic is japanese "rock", but the line is sometimes very thin to draw so... whatever. ;D