Oh my, I could go on forever when it comes to these games' soundtrack. I have to say they're very well-done (imo much better than OR/AS), but the battle themes especially shine. I prefer B/W's soundtrack as a whole, but nothing in the main game series even comes close to X/Y when it comes to battle themes. That's how good I think they are.
My favorites are the Gym Leader (don't really care for the Successor Korrina remix), Elite Four and Champion battle music, the Rivals' battle music, Team Flare's battle music... and last but not least, Mewtwo's battle music. My first reaction upon hearing that was "holy crap". It starts out very trippy, then you hear the Kanto wild Pokémon music... it was totally unexpected, but still mind-blowing in the best possible sense.
As for the various locations in the game, I really like Laverre City's theme and Kiloude Town's. The Kalos Power Plant is very good as well.
Another thing I really loved were the remixed versions of Emotions from B/W and Jubilife City from D/P/Pt. Both were very well-done and I actually like them better than the originals.