The most covered song ever?

Standing on a windy hillside on Saturday listening to a band, they announced that for their last song they'd play a 'popular classic' before launching into Europe's The Final Countdown.It struck me how many bands I've heard cover this song live - Faith no More, VNV Nation (as "the Swedish National Anthem"), Apocalyptica and the Cudworth Colliery brass band, amongst others, have all done versions when I've been there.

Which song have you heard covered more than any other?