L'Arc~en~Ciel is really what got me interested in Japanese pop/rock music, and, while I cited them as my favourite band for a while, I've come to be more obsessed with X Japan and Malice Mizer much more. Moi Dix Mois, on an aside, is a little spotty for me (as I liked their earlier albums, but the last few have been lacking). I love Gackt's voice, but I don't have much of his solo stuff. Speaking of voices, I also agree with SacredAngel13 that Mika Nakashima's voice is great. I'm also really obsessed with Yuki Kajiura's works. .hack//Sign is probably my favourite series she's worked on. Lastly, the Pillows and B'z are also enjoyable any time.