Black Sabbath basically invented the metal power chord and riffing style. Napalm Death sped metal up to the point where it became a whole new genre, we owe them the invention of the blast-beat. Although there were other "black metal" groups Bathory were the first to define what became the black metal guitar sound, a new way of thinking of guitar in metal where the guitar is almost an ambient texture. Slayer just set the bar for heaviness in general, Reign In Blood arguably still hasn't been topped for intensity.