Frank Sinatra "That's Life!" This is close to things that were passed off as rock in those days and would be "heavy" for someone like Billy Joel. Jim Morrison could have recorded this.
Don't know if these were already mentioned in this thread: Hey Willy - The Hollies Bad Bad Dreams - Bee Gees
I can't be sure about these, since I can't find a mention of weights in the lyrics, but... Jethro Tull - Fat Man Spinal Tap - Big Bottoms
This came up in an excellent Queen BBC documentary I watched last night, and once again, "Stone Cold Crazy" was the winner. I don't think it's even the heaviest on Sheer Heart Attack--I'd vote for "In the Lap of the Gods--Revisited," but maybe that's not enough of a song, per se.
I haven't heard every Richard Thompson song but I'd bet the solo on "Poor Will and the Jolly Hangman" (studio) might be the heaviest he's ever done.
I agree with you. In the documentary, they attributed it solely to May's guitar. I think "heaviest" has to take all the instruments plus vocals into account--but it's kind of a vague term anyway.