there wont be any definitive conclusion to the question, coz
1) The metal we like and the metal some other like, will never see eye to eye nor of the same level according to some
2) some metallers think their favourite metal band is more metal than what most metallers like and will never agree on the former's opinion of biggest metal band
3) we all are biased according to the metal we know and like and not those we havent know or like, but known to others and supported by others
4) the original question was not a proper question, without defining the definition of biggest, according to sex organ length, record sales, radio airplay, tour profit, most groupies, bigger than DMC, popularity in mainstream popular metal circle or stuck in forest, hidden away under moonlight and the bayan tree kinda metal etc
5) we only have personal opinions, but doesnt equal to same sentiments in term of global biggest.
imho, the biggest metal act in local context, is
this!