Well, if you look at anything (wins, championships, passing statistics, playoff appearances, longevity, big moments, etc), Tom Brady outshines Joe Montana. The only leg Montana has to stand on is the earlier era of rougher treatment for QB's, and there really isn't any way to know how Brady would have fared then, or how Montana would have fared in later eras.
So, it is basically up to subjective opinion when considering the era. Nobody really knows.
It's Tom Brady, in my opinion. I don't like him, mainly due to his countless temper tantrums on the sidelines, his cheating scandals with the Pats, and because he played for a division rival, but I think it is clear he is the GOAT.
Until, of course, Josh Allen takes that title in 20 years.