Yes, the Soviet Union did much if not most to bleed Hitler's Germany and finally overwhelm them, but in the end, I would have to give the most credit to Hitler himself. He made extremely poor decisions (such as invading the Soviet Union, for one) and even though his military experience was very limited, acted as though he was a brilliant warlord. In so doing, he led his country and his people to ruin.
I would give the most credit to the Soviet Union. By far the majority of German military men who died in the war were killed by the Russians. Only the Russians were seriously fighting Germany on land from early 1940 until 1943.
Of course, the US helped by giving weapons to the USSR and by the invasion of France and such, but the USSR really did the majority of the fighting.
Most probably neither the USA nor The USSR fighting the Germany by itself in the World War II would have been able to defeat Germany. AS a matter of fact, even both these countries together would not have been able to defeat Germany without the support of other countries that formed the group of allied countries. The defeat of Germany is the result of joint contribution of many countries. It is not appropriate give credit to any one country for defeating Germany. Countries fighting Germany in world war had different levels of resources and capabilities. But they all contributed to the fullest of their capabilities and they all must share equally the credit for winning the war.