Do we know yet whether the question is Conglomerate, Manufacturer, Brand, Family, Model Family, or Specific model. (and even at that: Brand/model recognition or actual units produced)?
In the latter of both cases, I'd say the Farmall H (300,000, just under 400,000 or over 400,000 depending how exact exact model is) is hard to beat - probably even worldwide. Else, don't even have a guess.