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?
This made me wonder what the oldest tune that would still be familiar to a lot of people today would be. Dies Irae is a good candidate. It's around 800 years old and is probably best known today from the 1980 version of The Shining, although I know it best from the Dr. Tongue stages in Zombies Ate My Neighbors.
The leitmotif for Palpatine is (loosely) based on the Dies Irae. And, AMAZINGLY, that leitmotif shows up in the happy singing of children during the parade scene at the end of the Phantom Menace. Because John Williams is a fucking genius.