It's a very interesting question to ask. Looking beyond the Pacific where the United States conducted the vast majority of operations against the Japanese, the war in Europe is hard to decide. While Britian did stay in the war for a very long time, they would have lost out if not for the Lend-Lease policy of FDR, which allowed supplies to be delivered to them. However, that should not take away from Britian's incredible resolve and fighting capability. After all they won the Battle of Britian and prevented Hilter from invading. America, though, was the predominate player in winning the supply battle in the Battle of the Atlantic. They contributed much to U-boat hunting technology and, coupled with the British were able to deliver supplies.
One also must consider that the British had ULTRA, which could decode the German Enigma machines. This proved invaluable in Torch and during the Battle of Britian.
Although the Sherman was a 'weak' tank to begin with, it recieved substantial field improvements which turned it into a pretty decent piece of machinery. You also have to remember the Sherman was a trade-off; four of them could fit on a transport in the spot of one tank the size of a T-34. With the exception of the German 88-mm flack/artillary gun America had very good long range capabilities. They also had the Superfortress bombers and the P-51 'Mustang' fighters. The M1 was the most reliable weapon on the field and the tommy gun was a pretty decent submachine gun. They also had the fifty-caliber machine gun, a novelty during the war, and, probably one of the greatest vehicles of all time: the jeep.
Whatever flaws were in American weapontry were made up by producing massive quantities. I believe that by the end of the war America had produced 2/3 of the weapons in the world.
Russia, although they fought brilliantly towards the end of the war, could not have completely defeated Germany without America's help. By keeping Britian alive German soldiers were committed to defending the 'Atlantic Wall' which drew veteran divisions away from the Eastern Front. Although Russia bore the brunt of casualties, I do not believe they could have beat Germany on their own.
I honestly believe it was a combined effort to beat Germany and that no nation could have done so on its own. America supplied a new fighting spirit, good equipment (and plenty of it), good soldiers. Britian brought veteran leaders, high morale, technological inovation, and experienced soldiers. Russia brought it's enormous size, will of it's people, and the T34.