Ill just say the arctic, because we all know Antarctica acts like the bloody World police, they have their hands in everything.
Them and their Fucking Pirate penguins
No one contributed the most on the allied side, as they were all working together, they pooled what they had to help with a common cause, who contributed most is a matter of opinion, not fact, every battle is a key battle, you cannot say oh we took caen, or we held out in Chambois so we had the bigger impact on the field, even the smallest fire fight can effect an entire campaign
for WW2 in general though i would say the Soviet Union because no one else contributed the lives of 8,800,000 to 10,700,000 soldiers and 14,154,000 to 12,254,000 civilians