Other than Tears, probably my favorite theme, and I honestly don't think the XIII soundtrack gets enough credit. Everyone seems to be overlooking it even though it probably has one of the more well rounded soundtracks in the series.
As for why I like this one? Idk, the feeling of urgency and a hint of sadness to it. It also helps that when first following this game, this theme stuck with me the most (only other one was probably KDD-0063...but that's because everyone in Arcade played K' on point...song will get stuck in your head after a while).