Which country was best about its' treatment of their citizens during the war?
I would argue that either the US or Great Britain was the best about its treatment of citizens. These were the two major free countries that participated in WWII without being occupied or invaded and these were, therefore, the countries that were best positioned to treat their people well.
In both of these countries, people's rights were generally respected. Of course, the US interned its Japanese-American citizens living on the West Coast. Outside of this admittedly major failing, there were no major deprivations of rights. In addition, the US in particular was well-off enough that its people did not suffer from hunger during the war. This was, of course, due mainly to the amount of resources available to the US rather than to any major decisions on the part of the government.
The treatment of citizens during the war was largely dependent upon the nature of the country (democracy or not) and the degree to which it suffered from the war. In both cases, Britain and the United States were better off than other countries and therefore treated their citizens the best.