In terms of assessing the long term consequences (not the "objective" merit) of their actions and policies, I'd actually say it's too early to tell exactly how the top four should be ranked. We still don't know what will ultimately become of the international scene in the wake of the Soviet Union's downfall, nor what will become of the country's social insurance programs in view of the ongoing budget and demographic trends. I'd say FDR gets the top spot for the 20th century, but what order Wilson, LBJ and Reagan should ultimately be put in is not entirely clear yet. How's that for a cop-out?
