i reckon nile are the heaviest band i've heard, they've got the worlds fasted drummer so i like listening to em Lol (most of his kicks are done with 1 foot and they're still faster than any other band i've heard)
http://au.youtube.com/watch?v=U4k2kFQn5WA&...feature=related
EDIT: porngrind?
I'm pretty sure any descent BrutalDeath/Grind band drummer can put him to shame for speed. Technicality, though. Neil Pert from Rush is still the best, by far.
Porngrind is bands like Waco Jesus, Lividity and such. Their lyrical subjects often deal with woman/porn. Basically, the scum of negativity in music. I enjoy their music, but I dislike their lyrical subjects. I just like their music for the shear instrumental reckoning.
It's possibly the heaviest genre out there.
Venom is awesome! But they are not the heaviest out there in this day and age.