Again, that would be every song ever. But I am going to go with "Fantasies Come True" from Avenue Q, because it's one of my very favorite showtunes but it's a (kinda) serious one that's sung by one of the more obnoxious puppet voices, so sometimes I try to show it to people and they can't get past that. BUT if I could play it, then I could just play it and sing it for people and they'd be like, "Oh, why, you're right, that is a nice song!"
...Well, maybe they still wouldn't like it. Maybe I'm just the only one in the world who loves this song. But at least I'd have tried all my options.
(And, if you're curious, here it is with lyrics. Original Broadway cast.)