I don't have a particular favorite. The boss battle theme that comes to mind when I looked at tihs thread was Force Your Way from Final Fantasy VIII.

When I was younger, I used to sing and hum incoherently to certain video game music and I realized about a year or two ago that those incoherent strings of words were somewhat sensical. I still sing to Force Your Way whenever I hear it: "You stay on top my mind, when you're happenin', you're happenin', now, so let's all go home to the land of the free."