I saw the title of the thread and instantly thought "Black Sabbath!" Now....heaviest song is a bit of a daunting task! How about "Into The Void?" Or "Symptom Of The Universe?" ................or 30 other songs
Kick Out The Jams...that opening line alone is heavier than anything some "metal" bands have ever done.
Maybe "heaviest" is not the best word in this case, but Pink Floyd never got any more "rocker" than this, in my opinion:
Some of these suggestions are laughable, if you're talking 'heavy'! Always thought Sabbath Bloody Sabbath had one of the heaviest, dirtiest guitar sounds around......
It's the heaviest song done by different bands. So each bands "heaviest" will, of course, vary wildly.