Obviously many countries were involved in world war 2 (hence the name "World War", but is there one country that we can sort of point our finger to and say that had it not been for them, the Allies would not have won?
The Soviet Union had the largest influence - a large majority of German casualties happened on the Eastern front, not in the West. However, it would be difficult to conclusively state that any one country "won the war". It was a combined effort.