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?
Amen Brother by The Winstons, more specifically the drum break on it. It's by far the most used sample of any song ever, and once you know of it you'll hear it everywhere kind of like the Wilhelm Scream in movies.