My favorite songs are the ones that play at Eterna Forest, the Old Chateau, the lakes and the lake caves, Canalave City, the upper levels of Mt. Coronet, the Distortion World, Routes 216 and 225, Snowpoint City, Eterna City, and outside the Pokémon League.
I also like some of the battle themes, like Cyrus, Galactic commanders, Giratina, Dialga and Palkia, and Elite 4.