I mean a song that's so fun/bouncy/happy/funny/sweet/sentimental... that you can't help but smile when you hear it.
For me, it's Tubthumping (I Get Knocked Down) by Chumbawamba. Just the name is enough to make me giggle, and the song just has this odd, charming quality about it that I can't quite describe....no matter how I'm feeling, this song is 99% guaranteed to make me just a little happier. I guess maybe it's like 'Walking on Sunshine', except way less annoying, blatant, and annoyingly blatant in its happiness. It's calmer, more like 'Here I am, perfectly content with my life, and really, really happy about that, but I'm not about to smack you in the face with my rainbow-puppy-hamsterpants happiness. I just feel like singing a little, 'cuz I'm happy. I mean, you'd feel like singing, too, if you were this happy, right? So just let me be, and I'll let you be. We cool? Awesome.' And then he struts away like the epic swagmaster he is (yes, I know I just totally trashed whatever rep I had by saying 'swagmaster', but you have to admit, it fits). Cuz he gets knocked down, and he gets up again! Ain't NEVER (nothing?) gonna bring HIM down.
So...yeah. What's yours?