Okay, so this thread is actually really interesting to me. When I fight, I find the "rhythm" of characters. Then, I find a song with that rhythm. For example, Cass fights at a rhythm of 115bpm. So, I decided that the song "Bad Company" at 114.5 bpm would be a good choice and so far it has been fantastic.
Also, almost every Sonic the hedgehog song works with every character.