I really liked the theme of Slateport. It was extremely cheerful and catchy, and always helped me to lift up my mood every time I heard it. I used to constantly fly myself to Slateport City in Emerald with my Swellow, and ride my bike around from here to there just so I can enjoy the music. Haha.
Vandenturf Town's music was another of my favorites. It was quite soothing, and never failed to make myself feel calm. I was also a fan of the Elite Four theme. Not the battling theme, though—just the one that you're able to hear before you start battling them. (: