Madredeus with the voice of Teresa Salgueiro
I'd say that accessibility, longevity, and cross-generational appeal are all properties that any greatest pop song would have. Not only should you be able to hum the tune yourself, but there should be a reasonable chance that if you asked a stranger, they'd be able to hum the tune too.
Or even, it seems, what they think is or isn't a pop song.
In my book, if you can buy it in Woolworth's or watch it on Top of the Pops, it's a pop song. The Rolling Stones and the ELO are definitely in.
So, while not the greatest song of all time, I do have a special place in my heart for: