I don't listen to japanese music very often, but I kinda like Aqua Timez, Otsuka Ai, Rokiez is Punkd', Tomofumi Tanizawa. ^^
ONE OK ROCK
KATTUN

two thumbs up!!!
Thanks to [I]Rurouni Kenshin[I], I've fallen in love with One OK Rock. :) Besides that, Arashi's my favourite J-pop group. I listen mostly to anime soundtrack, so it's a mesh of all bands and singers and groups. :p
Jrock
L'arc en ciel, One Ok Rock, Plastic Tree, Antic Cafe', Versailles, Vamps, Girugamesh, Lycaon, SuG, Heidi, X-Japan, Buck-Tick,LM.c, Gakido, Lancer>>Bee, A(Ace), Acid Black Cherry.
Some of what i listen to though I have a hard time choosing favorite :X Though Hyde of L'arc en ciel/Vamps is my forever favorite
Jpop
Yuya Matsushita, Gackt, Utada Hikaru, Koda Kumi, Kat-Tun, Arashi.
Don't think I have absolute favorite Jpop artist...
I really like Malice Mizer, ANTI FEMINISM, Royz, Kiryu, Penicillin, My Dragon, Danger Gang, X Japan, Aural Vampire and who can forget Gacharic Spin cx
Muss A (Suzy!) and Girl's Generation. :)
Ayumi Hamasaki, Anna Tsuchiya
I really like Scandal, Morning Musume, The Gazette, L.C.M, AKB48, C-Ute and Ayumi Hamasaki.
Gackt! No question there. I was a huge Malice Mizer fan i middle school when I got into Asian culture through anime and such, and Gackt/Malice Mizer was my first band haha.
For jrock fans here, check out a confession blog dedicated to jrock I made on tumblr !

Confess Your Jrock Here ~~