I am a big fan of J-rock, Visual Kei sub-genre(ish) Music. I am a fairly big fan of An Cafe, and I like Metronome. Also kinda like D and Nightmare. Utada Hikaru is pretty good, I had the KH theme song on my mp3 player for a while.. till it got run through the washing machine.. T_T.
I think that my favorite anime themes are #1: The theme to GIS SAC 2nd. gig (Though that's russian/english not japanese) And #2: The theme to Gunslinger Girl, which also is not japanese, but I still really like it.