OK, so these ones don't apply to Animorphs specifically, because I find myself thinking of them in great battle scenes in general when I'm reading, but the bigger Animorphs battles were no exception.
With Mad World, I've always thought that the haunting lyrics and melody make great background music for those times in battles when a character steps back and sees it all happening in slow motion while becoming overwhelmed with the horror of it all.:
[youtube]http://www.youtube.com/watch?v=4N3N1MlvVc4[/youtube]
Then there's March of Cambreadth, which is Celtic music at its most bad-ass. Maybe great for those couple of minutes it takes to morph. Yeah, I can totally see Rachel's face going bear, and flashes of various claws, teeth and bulging muscles forming to this song. Much cooler than the random-art-slideshow in this video:
[youtube]http://www.youtube.com/watch?v=eCrnF844_ww[/youtube]
By the way, I was afraid that this thread was going to be a bit more silly, but great job everyone one on a good variety of different music that is all so fitting.