From the one's I've watched, probably Double, OOO, and Ryuki.

OOO is the winner for me.

I'll cast my vote in for W. The music did well to set the mood without feeling very stale. I loved the episode where Phillip and Shotaro sang Finger on the Trigger.

Black, no contest.

The opening theme, Black Action, Blackhole Message, Long Ago 20th Century, all the incidental tracks... It doesn't get better than Black.

https://www.youtube.com/watch?v=C7pP1csDBD4

https://www.youtube.com/watch?v=8NCio6bJyM0

https://www.youtube.com/watch?v=9XKmmc36t6s

https://www.youtube.com/watch?v=cVdYHhwQy3E

^5

Hard question, personally I enjoyed 555's a lot and more recently OOOs and Gaim. Drive is proving to be pretty great as well.

I vote for Kiva too, with Wizard as a close 2nd.

Kiva, W, and OOO. Kiva for it's aweosme rock theme, W for it's sweet jazz backgrounds, and OOO for the many form-themes.

Blade and Kiva, followed by Black.

I liked almost all OSTs, but my favorite ones are Blade and Gaim. Though OOO has some tracks I also love to death.