The United States had the most influence in helping to win World War II. The two fronts we were fighting ourselves, especially the Pacific Operation which was almost solely a US effort. We also had the largest production number of arms out of any country in the war, that includes tanks, ammo, planes, guns, etc.
Russia is another good candidate, but they mainly had to concentrate on one front in the war, much of which was supplied by US supplies.
Ultimately, my answer is United States first, Russia running at second place.