I guess it is improtant to note exactly how one personally defines the term "heavy." For me heavy means hard, extreme, and crushing. All the heaviest bands are death metal IMO. Black metal isn't as heavy because the guitar sound is generally thinner and more trebly, and the riffing lacks the palm muting and bone crushing quality of death metal riffs. But something does NOT necessarily have to be fast to be heavy. For instance, the Nile song Sacrophagus is not fast at all but is extremely heavy. I think a lot of albums that were released a long time ago (70s and even early 80s) can't match up to more recent works in heavyness because they didn't have the crushing guitar sound, and metal hadn't evolved yet to include bands that are as heavy and crushing as they are today. Ahh, I'm rambling and I think I'll stop now.