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
Among my Favs
I'm a huge Jazz fan and Yuji Ohno is just my speed, this is a smooth tune.
It's a really cool song with a kickass baseline, Like Cowboy Bebop (Real Folk Blues is the business) I always wait for the song to end before watching the next episode.
These are mine favorite openings of all time
Aesthethic of the Rogue hero: Realization
Campione: Brave Blade
11 eyes: Arrival of tears
Yu yu hakusho: Smile Bomb
A certain magical Index: Psi Missing
Evangelion: Cruel Angel's Thesis
Sekirei: One winged vow
Eureka 7: Shounen Heart
High School Dxd: Innocent of D & Sympathy
Dragon ball Z: Rock the Dragon
Itsuka no tenma no Kuro Usagi: once
Problem children are coming from another world aren't they?: Black + white