Who is the best selling death metal band of all time?

This question can be a bit more ambiguous than you realise. Out of all the band which played Death Metal, Sepultura would be it. However, none of the albums they released during their Death Metal period would be particularly high grossing albums. Out of straightforward, true-to-form Death Metal bands, who haven't deviated away from it, Cannibal Corpse would be the band. As for the so-called Melodeath bands, they're irrelevant.