88. Hellhammer - Satanic Rites demo (1983)
Building on what Venom started, and adding a heavy hardcore punk influence, Hellhammer were a primitive early black metal band whose influence far outstripped their abilities; pretty much every black metal band on the planet can trace at least some of their sound back to this band.
There's not much here other than a band incompetently blasting away with as much speed, energy, and unmelodic noise as they can muster, but in 1983, this was a revelation (of doom). Slayer may have been more polished, but they couldn't touch Hellhammer as far as abrasiveness. They only ever officially released one EP before breaking up, but this demo--which is longer than many albums--is really their definitive release.
After the band's demise, several members (most notably Tom Warrior) would further develop this sound in Celtic Frost; but until the experimentalism of
Into the Pandemonium really set them apart from their early thrashing, I think their refinement of what Hellhammer laid down never quite justified that loss of primitive energy.