It's U.S.S.R., easily. Why is this even a question? The US (correctly, imo) let the great dictatorships bleed each other. The US was a financier but did not do most of the fighting. The UK is a tiny nation.
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.
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?).