I agree with SabintheFalcon. My vote has to be Britain. Yes, the occupied countries went through hell, as did Russia who fought an appalling war of attrition in the east. But the question is "who were the biggest badass" and to that question, I would say Britain. Why? Because they were the underdogs, should not really have had a chance, yet never fell.
No really.
Yes, sure, they had an empire/Commonwealth and all that, and a strong navy, but what is too often overlooked is that comparatively little of the manpower from those territories could actually be used, either because they were employed for local garrisons or/and peacekeeping, or because actual control of many Commonwealth armed forces lay with the Dominions. The likes of South Africa, Australia, Canada or New Zealand might loan an expeditionary force of a division or three (never very much), but could call them back at any time (so could not be depended upon for long - hard fighters though they were) and quite naturally the Dominions kept most of their strength for their own home defence. Britain's own native manpower was never numerous and actual military strength on the ground was therefore rather small. Furthermore, much of the resources from the empire were heavily disrupted by subs and raiders or lost altogether to occupation. For it's part the Royal Navy, although powerful as a whole, was spread thin across the world, vastly overcommitted, so that local strength was actually rarely much greater - and sometimes far weaker - than it's local adversaries.
So, despite appearances on paper, the British government in wartime did not have access to a wealth of resources, could not reliably turn to the empire for much manpower - and as was sadly proved, even the Royal Navy was not the reliable strength it appeared. And their own resources and ground forces were small. At the start of the war Britain had just six to eight divisions - and only two of these were Regulars. Obviously this situation was improved, but even at maximum Britain could only raise about 34 divisions in all, not all of them operational at the same time and with rarely more than 12 in any one theatre. The British army was SMALL.
With those facts in mind, Britain really shouldn't have had a chance.
And yet Britain, even after all of Europe had fallen to Fascism - never gave up. Churchill said he would never surrender, that he would continue fighting at any cost. And he meant it. Britain literally gave it's all to keep the war going, to continue the war to victory. Britain started the war as a superpower. It ended the war a bankrupt exhausted ruin and a political has-been, broken by the war, no longer important on the world stage as the two new superpowers, made by the war,duked it out. The admirable thing in all that is that in the Battle of Britain in 1940 the RAF fought off the Luftwaffe, thereby saving Britain from any real threat of German invasion. From that point on, Britain itself was safe. They didn't really need to fight on against Germany. Hitler had never really wanted to fight Britain, and so long as the USSR did not fall there was little chance of Hitler trying to invade again - and perhaps not even then. In 1940 or 41, Britain could have made a ceasefire with Germany and that would have been that. But they didn't. As SabintheFalcon touched on, Britain, despite poor available resources and a vastly inferior military strength, fought on; ever weaker, ever more desperate for resources, ever more outnumbered, but persevering, risking it all, to allow the Allies -and in particular the Americans - the time to build up strength, using Britain as a convenient base, and win.
It was indeed our finest hour. And we British have never been greater.
