Depends on what you mean by "contributed most". One reason for the high casualty rate from the USSR is their leaders' we have reserves attitude.
Considering how long and how much it took to defeat the Nazis, it's at least plausible that all three were necessary, or it would have taken much longer.
"Contributed" seems ambiguous. Are we talking about who took the most damage or who did the most damage?
I have found this most wonderful (if fairly lengthy) article, and thought you would enjoy having it brought to your attention.
It is so good, I think we should include it among the references in the "Politics is the Mind Killer" wiki page. But, before that, I submit it to you, and ask you: is there anything in this article that would warrant its exclusion from this site? I mean, besides the fact that it is about politics, and written by a notorious Social-democrat (And is that in itself grounds for exclusion?).