First of all to claim that any one thing or person or battle was the key battle, is a little extreme. I have heard historians say that some of the more important battles were those fought over Greenland. Hitler was using that area for a weather prediction outpost and such. Cutter boats were instrumental in the victory there. So, it wasn't truly one thing, but a bunch of things, which probably makes this whole discussion pointless, but I like it anyway.
The Russians may have had a large impact, but then you could say that it was because of the winter that they were able to turn the tide. The Russians were getting beaten at every turn until the winter started to hurt the Germans. Hitler made the mistake of allowing his troops to stay in Russia too long. So, I have a hard time saying it was the Russians who turned won the war. Besides if the Germans hadn't broken the non-aggression pact they had with Russia, Russia never would have gotten into the war. Stalin and Hitler had a deal, Hitler was dumb enough to believe he could fight a 2 front war and not have to pull out during the famed Russian winter. (Yes I know it was the Soviet Union, but it is Russia again and it is quicker to type Russia)
I believe the British did quite a bit, but in a more specialized manor. They were great at "special ops" they had spies and double agents that worked wonders all the way to Berlin. It is my opinion that Montgomery wasn't all he thought he was and got alot of credit because he was one of the highest ranking British officers and he seemed to have a good PR person. The Russians, British and American split most of the kudos amongst themselves, but with the cold war coming on, the Russians weren't given much credit among the westerners.
The US did a great deal, but they didn't really have the "hardships" that the British and Russians had. Besides Pearl Harbor and Midway there weren't too many attacks on areas that were held by America. So, the American "War Machine" was able to function without dealing with having to rebuild buildings and so on.
I think the greatest contribution was made by Hitler himself. Between not truly trusting his aides to holding back German tanks from Normandy because he believed there was to be another attack (the main attack) further South. Also, not unleashing Rommel on his western front was a mistake in my mind as well. He should have given Rommel complete control. Staying in Russia for the winter and I believe he got a late start on that whole assault anyway, is a mistake that he should have known not to make. Hitler seemed to pull the reigns in on his men when things got a little rough, which hurt one of the main premises of the blitzkrieg, that local officers make the decisions that effect them most. That generals make overall plans and officers make local plans. In the end there were troops that were waiting north and south of Normandy that were waiting for the call from Hitler to move or counter-attack. The British counter-intelligence units "made" Hitler believe that Norway and the Paile de Cailes were major targets, much bigger than Normandy. (Please forgive the spelling and I am not 100% sure about the exact locations -- but somewhere north of Normandy and somewhere south of Normandy). So Hitler held back troops through the end.
There were too many contributions made to really identify the biggest, however Hitler did far too many things for him to ever overcome to win, but I believe the US made it a shorter war.
And let's not forget the French who added new meaning to the word surrender. Again the spelling is difficult but the Majenough line. It was a line of cannons and other obstacles built before the war to hold back the Germans. It was built along the border of France and Germany, it cost billions in todays dollars and the Germans were in Paris in 2 weeks anyway, with French troops warming up their white flags.