I used to enjoy "Jump Up, Superstar" but I heard it so much I ended up getting sick of it.
I like most of the ones mentioned so far. One that I have yet to hear is So Nah, So Fern.
The soundtrack in this game was abysmal, but this song (and its instrumental version) really stand out.
I was going to mention Ashley's Song, since I've heard it in Smash quite a bit. Turns out it is 15 years old.