If we're talking Thrash aswell as Black Metal, then I must mention a few of my favourites. Obvisously the "big four", although I don't think Metallica deserve to be included in that anymore, maybe Anthrax too? So, Slayer & Megadeth for sure.
To me, there was another "big four", a better one at that too; Overkill, Testament, Exodus & Metal Church.
As for strictly "Black" Metal, then I'd have to go with Venom, Bathory and then Celtic Frost for being the true groundbreakers and innovators of that particular genre.