Best Song in the World

Back in 2002 Tenacious D sang about how they had encountered a demon, and to save their souls sang the first thing that came to their mind, which was the best song in the world. They since forgot the song, and made a tribute, which sounds nothing like the best song in the world.

11 years later One Direction seem to have got hold of the best song in the world, as they sing about dancing to the best song ever. They too forgot how the song went.

What is this song? I want to hear it. Are they talking about the same song? Everyone seems to forget what the song is after listening to it...does it really exist? I need answers!