i hate this question it's been asked over and over again, by people and televsion, the answer to your question is, what ever your best song in the world is, is the best song in the world.
Mine would probably be American Pie, as I love a good sing along, but if you asked for 20 favourites, I could come up with them easily!! My favourite depends on my mood at that time.
this is a hard question to answer! thier are so many great songs out thier. it's almost imposible to answer! now if you said what is the best song lynard skynard did, i would say all thier stuff. madonna, that would be all her 80's stuff. see you can't narrow it down to one song, even by a group!