Population disparity between USSR and Germany

I'd disagree. I think the Wermacht was not quite as good as you give it credit for being, certainly not the "best fighting army" in history. Japan I think you give too little credit, they did after all inflict the greatest defeat Britain ever suffered.

 

I also disagree :

Such thing as "the best fighting army in history" does not exist

The WM did not lose because of Hitler

War is not about being even in terms of men and material

Not only did Japan inflict a big defeat on Britain,but also on the US .

You can't say that the Japanese army was laighable compared to the WM : they did not fight against each other .

 

You're completely right with regards to the Japanese defeat of US forces, they were decisive. I included Malaya/Singapore because it's generally accepted as the biggest British military defeat, in history. Singapore (the impregnible fortress) fell to a Japanese force barely 40% the size of the British/Commonwealth force. Chirchill himself called Malaya/Singapore as the "the worst disaster and largest capitulation in British history".

With regards to the US military, while the Japanese did deliver a huge defeat, and beat them every bit as soundly as they beat the British/Commonwealth forces in Malaya and Singapore, it was not the US's greatest capitulation and it is debateable if it was their greatest defeat in history. That honor or dishonor is debated as being between the Philippines or the US Eighth Army's rout by Chinese Forces in Korea, as to which was the larger disaster. The Philippines resulted in the largest surrender of US Forces since the American Civil War and to a Japanese force significantly smaller than the allied forces. So it was a huge defeat and humiliation, but not the US's largest capitulation in history. The Eighth Army defeat in Korea resulted in the longest retreat in US Army history. If you include the initial Chinese attacks at Onjong (primarily ROK forces) and Unsan (ROK and US 1st Cavalry Div) where the 8th Cavalry Regiment was virtually destroyed and the 5th Cavalry Regiment badly mauled, instead of starting with the fighting around the Ch'ongch'on River, it's an even bigger defeat. Since the first two battles were really part of the the same ongoing fight I think they should be included. I think that the US Army seperates the earlier fights to minimize the magnitude of the defeat.

 

Well St. Clair's defeat was in some ways the worst ever for the US Army. His force which included most of the regular US army at the time suffered something like 95% casualties. See:
http://en.wikipedia.org/wiki/St._Clair%27s_Defeat
http://www.history.army.mil/books/AMH-V1/ch05.htm

As for the IJA I find it rather hard to rate them. Equipment wise they certainly weren't up to the Western armies particuarly if you are looking at armor. On the otherhand their moral at least early on was very high and their discipline was incredible. From what I've read of Gaudalcanal however they weren't up to Marine standards in weapon handling from the bayonet on up. They were also vetrans who initially faced forces that weren't and who had in many cases less than stellar leadership.

The Japanese military also suffered at mid and higher leves from some incredibly vicious infighting. While the army and the navy at times seemed to consider each other the enemy rather than their opponent in the field there were also internal issues that were nearly as bad from what I've read.