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?
While not what one would think of when they think of songs that survive hundreds of years from now, the only song I can think of that's not a folk song that's both archived and hummable (and actually has a tune, so that excludes pop songs)...... is the Pokémon theme song. Go up to anyone and say in tune that you wanna be the very best and someone's gonna ask "like no one ever was".
i have this thing where when i'm focused, but switching tasks, i'll click my tongue but it's always the tune of nick nick nick n'nick nick nick o lo dea onnn