After the Soviets, the largest contributor to Germany's military defeat was Hitler himself. He fancied himself a military genius, but he ordered the Wermacht into military disaster after disaster.
The other think I never realized about Hitler until watching some documentaries recently, is how much the guy was obsessed with outsized and useless military weaponry. He had Germany waste immense strategic resources on trying to build "wonder weapons", most of which were failures, and none of which would have offered anything other than a nominal tactical advantage at best. Those were resources Germany could have used to much better effect in defending and consolidating their Reich. At think at some point the Soviets and Anglo-Americans began to realize the Hitler was Germany's own worst enemy, and a more capable General would have presented the allies a daunting task.
I think you have to be very careful making these sort of assessments in that your view can become revisionist. Who contributed the most to winning a war can be a bit of a moot point and what if scenarios, such as, what if Hitler had defeated Britain before invading the Soviet Union, are just that. What if’s.
Now it is true that the Soviets did kill the majority of German Soldiers, contributed more soldiers, and sacrificed more lives. The later, though appalling, isn’t necessarily an indicator of a contribution to victory as much as a measure of the Soviets disregard for the lives of their soldiers.
In terms of Empirical numbers the Soviets contributed the most soldiers and killed the most German soldiers and destroyed the German Army in the field.
The Americans contributed half the entire war materials production during WWII. That’s including Axis production. A staggering number. The British Empire produced another 25% and the Soviets 10%. The Axis powers only contributed 15% of total war Production with Germany responsible for about 11-12%.
So in terms of contributing materials the US contributed the most and the British Empire still contributed more in materials than all the Axis Powers combined. It’s staggering to think about but the Allies produced 10 to 1 over the allies and gives context to the commercial dominance of the world then ( and now) by the English speaking people’s.
To further emphasize US and British contributions the progress the Red Army made in the field would simply not have been possible at the rate they succeeded without the material contributions of the US and British Empire from lend lease or the massive destruction inflicted on German infrastructure and war production by their combined Air Power which crippled the German Army and made the Red Armies success in the field possible.
So the reality is that Germany and the Axis Powers were defeated by a coalition of allies who made massive contributions where they had available resources and shared those resources in order to assure the defeat of the Axis Powers. The result was the Axis Powers were swept away by the Allies who had a vast superiority in man power, materials, technology and money by an order of magnitude.
So the question of who contributed the most to defeating the Germans and Axis Powers is not only complicated but a bit of a Moot point.
What really matters is who were the Victors in WWII? Clearly Germany and the Axis Powers were the losers but for the major Allies the British Empire and the Soviet Union the victory was a Pyrrhic victory as the cost of victory was so great that it cost both their Empires and neither Empire now exist and both the UK and Russia are no longer Super Powers.
What is clear is that the United States of America was the clear winner and overwhelming beneficiary of winning WWII even though other Allies contributed substantially to winning the war the vast majority of the benefits of winning WWII have belonged to us Americans.
To our credit us Americans have put those fruits of victory to good use. Though warfare and our own tendencies for war still exists the numbers of people killed and displaced by war are at all time low historical levels and human achievement, productivity, and standards of living are the highest now in all of human history. This era of post WWII peace and prosperity brought back from the fringe of human destruction will be known historically as The Pax Americana and it is our nations greatest achievement. Though one we didn’t achieve alone.