As in, what song do you listen to when playing a particularly tough level or area? That music that triggers a determined reaction out of you...
The unknown passage theme is really awesome, but I joined someone in It Came From Below and decided to listen to Final Test during it instead. (film context if you're interested)
Another one I like to use during arenas is this (Is a download link, because I don't know the title of the song. I know it's an arrange and vocal swap of an Alstroemeria's song, but they release a different arrange of it in like every single one of their albums so I've no clue which one it is) It's a tad short though so it's only good for a single round of an Arena.
ECHO is a song I listened to when I first completed Flandre. And I still use it because it's good all the way through.
Progressive songs (example, Two Steps From Hell - Dark Ages) aren't optimal IMO since they need time to build up and give you that kick, time you may not have.
Not joking- Crush on You - The Jets