We also can't forget the Guilty Gear 2: Overtureremix. (Give it a minute, it has a longer intro than the XX version.) It manages to pull off the feat of switching the melody from electric guitar to violin and make it arguably even cooler than the original.
It is no longer better than the original thanks to the GGxBB Music Live 2011 version, which merges all three versions of Holy Orders together.
The Remix by A.S.H. (the same guys who did the Sound Alive renditions) from the Bonus Disc 8 of the Complete Sound Box. This and other remixes from the Bonus Disc 8 will play if you're fighting against Order Sol.
Conclusion, the ORIGINAL rival battle theme music (Sol vs Ky or Millia vs Zato). Looking back, even with PS1 qualities, that song still KICKS BUTT FROM HELL.
The second game in the series, Guilty Gear X: By Your Side, now feature distilled rock music, and it's awesome (Note: in it's original arcade release it had synthetised music instead of pre-recorded. Not that it was bad. The ones linked below are from the Hard Rock Tracks version of the console versions):
The Original, the new theme of Dr.Baldhead, who now goes under the name of benevolent legit crazy doctor Faust.
The music from the third game in the series, Guilty Gear XX/X2: The Midnight Carnival, are the ones everybody most familiar with. Your Mileage May Vary on whatever the remixes of the previous songs are good or not, but the themes made specifically for XX are awesome:
The series most infamous character, Bridget has a nice theme called A Simple Life.
While composed by Masaomi Kikuchi (who already had scored some tunes in Isuka) instead of Daisuke Ishiwatari Keep in Gates, the theme of Canon Immigrant A.B.A. is still good in it's own right.
Get Down To Business, the theme of Order Sol. This theme screams "You are about to get royally fucked up." Then it turns into something out of a western tavern-brawl scene.
The Spin-Off game, Guilty Gear Isuka, while very unbalanced, still got awesome music. In addition to the series main composer Daisuke Ishiwatari, there also Tetsuya Ohuchi and Yoshino Kusano, who scored various menu themes and other misc. music, and Masaomi Kikuchi, who scored A.B.A. theme and half of Boost mode themes.
Along with A.B.A., the console versions featured the Boost mode, a Beat 'em Up like game which consisted of five stages, and of course, five new themes:
The first stage, the farmlands, has Fille de Vent theme.
Diva, Valentine's theme in her final form. Too bad it always gets interrupted by illusions.
This one doesn't even need explanation. Re-Coming.
Ass-kicking has a theme song. That theme song? Starchaser. The fact that its Sol Badguy's theme on the Korean OST is just the icing on the cake.
While we're on the subject of the Korean OST, its version of Midnight Carnival happens to be not only a fine piece in its own right, but easily the best song of all at reminding you that you are fighting a Really Final Boss.
The songs are live showstoppers as well, as evidenced by the Sound Alive album. Some believe that the live version of 'Existence' far and beyond outpaces the original game version.