Hrmm..
To start Yousei Teikoku
Otherwise..
Nightwish
Also like techno or opera or classical depending on my mood
You could be my music BFF. Seriously.
Nightwish was my favorite band until I discovered Yousei Teikoku (Last Moment, Wahrheit, Pray in the Darkness, Valtica and Alte Burg are my absolute favorites). Nightwish fell off my radar after Tarja was replaced by Annette, mostly because she (Annette) doesn't really give the same feel to the songs that Tarja did. Still love their older albums though.
Used to be really into old school rock/metal (Bon Jovi, Queen, Guns 'n Roses, Iron Maiden, Metallica, Def Leppard, etc.), but I don't find myself in a mood to really want to listen to those anywhere near as much as I used to.
Big fan of compositions and songs by Toshiyuki Omori, Jun Maeda, Togoshi Magome, and Shinji Orito.
Obligatory fan of Daisuke Ishiwatari.
Random Japanese singers I'm a fan of: Kotoko (no idea why), Sachi Tainaka (love her voice), and Lia (ditto)
Random songs that I absolutely love: Higurashi no Naku Koro ni, Katayoku no Tori, Shiroi Kisetsu, LeMU -Haruka na Lemuira Tairiku-
Random bands/groups I like: Trans Siberian Orchestra, The Crystal Method
Assorted anime/game/visual novel music tends to make its way into my playlists, but as there really isn't any consistency to it as far as composers go (not counting those already listed, anyway), and there are far too many to list, this is all the mention they get.
I'm not a fan of most current American music at all due to most songs having (imo) fairly boring/uninspired melodies, bands being too heavily dependent on vocalists who can't back up that dependency, and instrument choice (synthesized or otherwise) practically being uniform.