There are some hints in YouGov's poll that memories of the Cold War may be a factor. Americans aged 18-29 were the age group most likely to credit the USSR, though they still prefer the US by 36% to 16%. However Americans 65 and up, who would have mostly come of age during the heart of the US-Soviet rivalry, pick the US over the Soviet Union by 66% to 9%, a much wider margin.
Or maybe Spain, by staying out of the war and keeping Hitler on read they made it possible for the allies to actually gain enough power. Winning through pacifism.
Who did the most? I'd say probably the USSR by numbers. The US was a huge driver of the war effort, and used a blend of hard and soft power to push. The UK and (Free) France were necessary resistance and emblematic of the other western efforts. The Battle of Britain can't be understated, they were a huge stopping block. ANZAC forces weren't as prevalent in WW2 as they were in WW1 - they weren't the leading force, but they were present and earned every ounce of respect. In theory, Japan could even be on here as a catalyst - even though they were allied to the Nazis and did many horrifying things, had it not been for Pearl Harbor, the US wouldn't have gotten involved.
But really I think it is either the USSR or the USA, with the UK a distant but necessary third.