Your Favorite Anime theme song

[Singularly Datarific]

This my be cheating a bit for the opening choice, but I'm going to go with Ask DNA by the Seatbelts, which is the main theme for Cowboy Bebop: The Movie. It's just...so damn catchy and easy to sing along with.

I thought that was Tank.
Anyways, that and the Last Exile theme:

 

[-BloodRush-]

That'd be the Hellsing theme song, hands down. Perfect groove.

this or the cowboy bebop the OP posted.

 

[Zosephine]

Again by YUI, one of the openings from the new Fullmetal Alchemist anime:

[link]http://vodpod.com/watch/1743824-again-fullmetalalchemist-brotherhood-opening-yui[/link]

 

[LogicNProportion]

Cowboy Bebop's Opening
Hellsing - EVERYTHING (TV Series)
Naruto's GO! Opening
Code Geass' COLORS
The Big O's ending "Forever" always gets me where I live XD
Neon Genesis' opening
Beat To Death Angel Dokuro-Chan's 2nd Season ending theme was cute to clusterfuck extremes

AND FINALLY

HARE HARE YUKAIIIIIIIIIIIIIIIIIIIIIIIIII!

 

[AHeroNextDoor]

Anything by Yoko Kanno is epic. I'm proud to say I own all the music of Ghost in the Shell (both the movies and the television episodes).

That being said, my favorite opening for an anime currently is a split between Wagdug Futuristic Unity "Systematic People" (Kurozuka, really good anime!), or Eiko Shimamiya's "Higurashi no Naku Koro Ni" (Higurashi no Naku Koro Ni, a really twisted anime that I still can't wrap my head around).

 

[Cado_Deus]

HOWLING by Abingdon Boys School, intro for the first 12 or so episodes of Darker Than Black