Here's a couple for now...
One piece, we fight together
Black cat opening
and probably the opening for Guilty crown season 1, the first few seconds. I'M YOURS
Difficult from One Piece. The theme is just so threatening and fills you with such foreboding, not to mention that it is used in the best scenes & is the theme of Ouka Shichibukai.
Orochimaru's theme from Naruto. Just so vile and detestable, plus it made the first Naruto series for me:
Another sinister one, and its various variations: Dante from Fullmetal Alchemist (Really set the mood for the FMA series).
The sword that protects - Rurouni Kenshin of course:
Call Me - Cowboy Bebop
Number One - Bleach
The rest is made up of the Mew Mew Power soundtrack (weird, i know; still my first anime though),
Master from Code Geass,
Tear drops to earth from Black Lagoon,
Almost the entire Cowboy Bebop soundtrack,
Estate of an Assassin - HxH
Eiyuu Ou - Fate/Stay Night
After decades the 'blackout' rule may be coming to an end.