This is sort of a question without a good answer, at least in the way it is here, its broader than is really helpful, and begs for generalizations. That being said and in the absence of further clarification or follow up questions from OP we can tease out two points worth discussing here.

The first being that if we take it to mean which nation contributed the most monetarily, materially, and organizationally to the defeat of Imperial Japan then the answer is essentially without reservation the United States. The US was by far the most active player in a war fought across half the surface of the earth from Sydney to Anchorage and San Francisco to Rangoon. There was not a front in the war that American fighting men and even more American supplies and equipment were not deeply felt and present. While the death blows of a devastating submarine campaign, reduction or capture of island bases, and eventual bombardment and blockade of the Home Islands was primarily an American show with contributions from many of the other Allied powers.

But that isn't the whole story. The answer changes or at least becomes more qualified if we ask what people tied down the bulk of Japan's fighting men for the longest, and which both drained resources and helped bring about the chain of events leading to the cataclysmic Pacific War. Then we have to turn to the people of China. For 12 years in increasing intensity they bore and fought against invasion by Japan in some of the most devastating and barbaric fighting of the war. And until the final act of 1944-45 the majority of the Japanese Army was deployed to the mainland seeking to control their bloated but unstable empire. China was a bleeding wound for Japan on a scale Spain only hinted at for Napoleon. However the people of China were not organized or loyal to a single government. While the Nationalists of Chiang Kai-shek were the largest single organized one they had emerged from among many competing warlords, and control across much of the unoccupied country was tenuous. While Mao and the Chinese Communists were still a force to be reckoned with and the peace between them to focus on the Japanese was never easy and with wary eyes to the future and a resumption of the fighting never far from the minds of either side. And the human toll reflects this state of total war, with most estimates placing the Chinese war dead at around 3 million, and civilian dead at upwards of 12 placing the Chinese people right up there with the peoples of the Soviet Union with the scale of the human tragedy the war brought. An aspect of the war that for various reasons gets little page or screen time in the West. And of course Japan's increasingly imperialistic adventures culminating in invading China are what soured any last hope for peaceful geopolitical coexistence between the Empire and the Western Powers.

But in general it depends on how you want to take the question the US led a fight across half the world and was felt in every corner thereof, while the Chinese suffered the harshest blows of Japan for over a decade and put Japan on course for its own destruction.