I define heavy as "brutal", these bands should overcome your senses with an onslaught of pure aggression. I don't think bands with burping style vocals are that heavy, sometimes melodic bands (At The Gates) can be even heavier than other styles of metal, Doom bands are only heavy in a one-dimensional way unless they are death/doom, I think fast bands can attain a more brutal sound by combining heavyness with speed,
the bands I would consider the most heavy are bands which have a primitive sounding attack with fast, precise distorted riffs, very evil vocals which are well articulated and evil sounding, and these bands are usually consistent in their songs, and do not "let up", bands like early Entombed, Krisiun, later At the Gates etc...