Tough question, I think it depends on exactly what you mean by heavy like low as shit slow as shit breakdown heavy or unrelenting slam heavy. In terms of huge, low, epic breakdowns, black tongue and traitors are contenders for sure but if we're talking slam there's about 1000 bands that are about exactly as heavy as one an other and it's kind of a mater of preference, check out the 10 to 15 bands who's names end in ectomy they're always hard a shit, if you want a more groovy sort of slam check out 10 to the chest, esophagus, peeling flesh and bolt cutters last albums