If you don't mind an old-guy perspective. Before anything was called hard rock or heavy metal there was 'heavy music'. Even though it began being called heavy metal by some after "Born to be Wild", you most often heard it referred to as heavy music in underground radio and rags from early 68, possibly Blue Cheer's "Sumertime Blues" being the first, thru early 70's. Blue Cheer, Iron Butterfly, Vanilla Fudge, Deep Purple, Cream, Led Zeppelin was all referred to as heavy music. But I think the first Black Sabbath album pretty much nailed it down. That was no mistake heavy music. But the point I'm trying to get to is, heavy music wasn't frantic and fast with every hole filled with something (holes in the music, not the... uh... anyway), but normally was medium or slow paced, with a lot of space, which make things sound bigger, contrary to how some go about it.
Anywho, enough history. For a truly heavy song check on The Move's "Brontosaurus, and for a bass-oriented heavy album, The Move's "Message from the Country".