In 1945, most French people thought that the Soviet Union deserved the most credit for Nazi Germany's defeat in World War II - even though the Soviets did not play much of a role in France's liberation, relative to the US and Britain.
By 1995 and 2004, however, the French had changed their minds, and were crediting the US as the biggest contributor to victory in Europe (survey data from the French Institute of Public Opinion)...
Assessing the "biggest contributor to victory" in a rigorous way is exceptionally difficult. They tend to devolve into comparisons of counterfactuals, and the truth is that nobody has any strong idea how the war would have turned out absent US involvement, or if the German-Soviet non-aggression pact had held, etc. Soviet Union's successful resistance of Nazi invasion and subsequent reclamation of Eastern Europe was the most important of many factors in defeating Germany. As historian Richard Overy Explains In His book Why the Allies Won :
If the defeat of the German army was the central strategic task, the main one was the conflict on the eastern front. The German army was first weakened and then driven back, before the main weight of Allied ground and air forces was brought to bear in 1944. Over four hundred German and Soviet divisions fought along more than 1,000 miles. Axis divisions between 1941 and 1945. The scale and geographical extent of the eastern front dwarfed all earlier warfare. Losses on both sides far exceeded anywhere else in the military contest. The war in the east was fought with a ferocity almost unknown on the western fronts. The battles at Stalingrad and Kursk, which broke the back of the German army, drew from the soldiers of both sides the last ounces of physical and moral energy.
If you are looking at the human toll of the war, the Soviets clearly incurred the heaviest losses. Tony Judt's Postwar cites Estimates Suggesting There Were 8.6 million Soviet Military Deaths and over 16 million civilian Deaths in World War II. The US lost 418.500 Military and Civilians in all theaters of the war - still a staggering figure, but not on the scale as Sami Soviet Losses. Of course, it's possible - and highly preferable! - to contribute to the success of the process. But it's worth reflecting on just how massive the sacrifice the Soviet people made was.
"The victors are those who write History. It is this one that is written in our school books, not the true History as it unfolded, but a History that caresses the camp of the winners. History has ceased long ago to be the sum of the humanities today it belongs only to a handful of individuals. "