Which group benefited most from the reformation and which was most impacted in a negative way?

The German princes, dukes, and kings benefited most, it fragmented Germany beyond all repair, and permanently emasculated the Holy Roman Empire of the German Nation, the only possible form of political unity for Germany in the Middle Ages. For that reason, I would say the German nation as a whole were most negatively impacted, as the Reformation helped entrench the centrifugal tendencies in German politics, and delayed national unification for over 300 years. Of course, the Catholic Church got screwed over, too, but that goes without saying.