So there was a vote for J-rock bands after all...
Anyway, I'm going to go for X Japan. They were groundbreakers in their time, and most probably paved the way for the rest of J-Rock and visual kei to follow. And they made some really fantastic music too, from heavy speed metal bordering on insanity, to ballads that were intensely emotional and powerful.
It seems like I'm running across X Japan related stuff on the forum a lot today. First the hide museum thread, then the Gackt and Yoshiki collaboration, and now this one.