My answer to this changes basically every day. It's Cyndi Lauper's Girls Just Wanna Have Fun. It's Prince's When You Were Mine. It's Dire Straits' Romeo and Juliet. It's Baba O'Riley. It's Gimme Shelter. I'm not really choosing based on grand musical quality, or even depth of stuff I could pull from the track. I'm also not choosing for a piece of music that could replace all the others; these are just some songs that right now make me very happy when I hear them.
My best stab at an answer might be Simeon Ten Holt's Canto Ostinato, a lengthy piece for multiple pianos in which the musicians move seamlessly from one short phrase to another, repeating them as many times as they want. You can't accurately predict how one recording will be different from another, but it moves like shifting water from one section to another. There are bits of that piece I'm familiar with, and hearing them arrive is always a surprise and a delight.
