Greensleeves is almost 500 years old. I'm sure there were other very popular songs when it came out, but Greensleeves had to staying power to still be here. What do you think is today's Greensleeves?
Happy Birthday has the kind of universal recognition you'd be looking for. Maybe in 300 years there'll be a lyrical shift towards something more interesting. I know multiple versions of Greensleeves. The Cuckoo is the other song that I can think of with a long history. The wiki article doesn't fully capture it. I'll stick something in here later.https://en.m.wikipedia.org/wiki/The_Cuckoo_(song)