I'm not even sure if I agree with the concept of "greatest song of all time". I mean, I can name tonnes of perfect songs, but they are all perfect in different ways. Is a perfect metal song better than a perfect folk song? (In fact, don't answer that, 90% of forum members)
However, I'm going to put some things that people are less likely to choose but definately deserve a place, because I'm pretty sure this forum will give way more attention than is due to metal, classic rock and videogame music.