For me, FREE 1 stands out the most. It's the first piece of music that plays once you leave the dojo and set off on your quest for revenge. It sets the mysterious tone perfectly considering the fact that Ryo doesn't know why his father was killed and is currently unaware of what he is getting himself into.
It's also peaceful but sad, as if Ryo has come to terms with what has happened and is no longer in shock but at the same time still deeply upset and depressed.
I'm probably overanalysing it, but I can't help but think that every time I hear the song.