The posts so far seem to associate Heaviness with a degree of brutaility, personally I do not think brutality and heaviness have much to do with each other. Heaviness for me is something slow and crushing, so I would consider doom the heaviest genre or perhaps some old school DM. As for bands, I'm not sure, perhaps Miasma, they have some extremely heavy songs.
Kind of agree with this. Not too many of the "brootal" death bands would make my list on heaviest bands. Their music is often too busy, too many riff changes, too much tinny snare banging, etc... Hardly heavy. I suppose the one exception for me would be Severe Torture.
But, anyway, heaviest stuff follows:
Obituary (Slowly We Rot and Cause of Death are easily two of the most bone crushingly heavy releases EVER)
Incantation (especially Diabolical Conquest)
Mortician (go ahead, laugh if you will, but before you do, please give House By The Cemetery another listen. VERY heavy.
Grave (early stuff, with the original singer)
Disgorge (their stuff on the split with CBT, Goremassacre Pervosity I think it is called, is some of the heaviest, thickest grind you will find)
I think, just for fun, I will add a couple black metalish bands to the mix:
Black Witchery
Blasphemy
Archgoat
