It is hard to pick out just one, but here's my list of favorites.
-Imperial K.K. - It's Japanese like style, and its pretty catchy
-Rockin' K.K. - It's upbeat and fast tone
-K.K. Ragtime - It sounds pretty classical, and it was the first K.K. song I've ever heard.
-DJ K.K. - It sounds like I could have dance party with it.
-Steep Hill - It sounds pretty relaxing.
-Spring Blossoms - Japanese feel and upbeat tone.
-K.K. Island - Nice music to relax to.
-Bubblegum K.K. - Upbeat, happy tone.