Rain by Madonna and Girls Just Wanna Have Fun by Cyndi Lauper
Both they were significant at a significant time and they made me happy, then. Funny how that carries over.
Another one that has a similar but slightly different impact is Suzanne (the Leonard Cohen song as sung by just about anyone else). That conjures more of a dreamy, pensive mood, though, not pure joy.
I've just started listening to Sing You Home by Jodi Picoult (audio book) and the narrator talks about the impact of music on the brain and memory and mood. It has made me interested to learn more about that.