There is a lot of music in this game that I like. I like the battle music in this game, because when I hear the music again, I immediately relate it to my previous experiences when I hear that same tune.
My favorite is the theme of Galactic's leader, which you only hear a couple times in the game. I also like Dialga and Palkia's theme, which are exclusive to them, so you only hear it once. This makes it truly special. I actually do like all the battle music, but I like those most of all.
Cynthia and Palmer's theme I like too, and it's too bad I can't find a complete MP3 on it.
Other BGM that I like are Snowpoint City's theme, which fits the theme of the town well, and the Pokemon League's theme, which I can chill to. I'll leave it at that, because no matter what I say, I'm going to miss another favorite, anyway.