IMO it's too hard to say, especially if you include bands that released less than three albums or death metal albums. And often bands with a more extensive catalog have released rather weak albums along with their classics (ie Morbid Angel, Death, Unleashed, Grave, Incantation, etc.). Gorguts, Immolation, Monstrosity, The Chasm, etc. seem to be some of the most consistant bands, though even with them not every album is close to flawless.