The first song I got in Animal Crossing was K.K. Ballad, given to me from my best friend, it's been my favorite ever since. I also really like Stale Cupcakes, Bubblegum K.K., K.K. Love Song, and Forest Life.
The first song that he played for me was K.K. Disco and it's still my favourite so far! I haven't heard all of them yet, but I will probably always have that one playing in my house, haha.