If there is one country we could point to and say "if it wasn't for their effort Germany could very well have won". Obviously there is no way to be sure about this, just would love to hear opinions along with facts to back them up.
I'm going to tell you this right now, OP, you've opened up a big can of worms of opinionated answers, and a huge string of [deleted].
But not necessarily the biggest impact, but the least appreciated if definitely Canada. I'm going to skip over the 'famous', for lack of a better word, ways Canada participated (e.g. D-Day, Bomber Command, Battle of Britain, BCATP, etc), and focus on the lesser known aspects of Canada's contribution to the Second World War.
One of easily the least known contributions, even in Canada, is Canada's contribution to the S-E Asia Campaign in Burma. In Canada, the battle of Hong Kong is an extremely famous aspect of Canadian Second World War history for many reasons, but Canada in Burma has largely been forgotten. No. 413 Squadron was posted in now what is not modern day Sri Lanka in 1942, and flew recce, anti-submarine, and convoy escort missions using Catalina Flying Boats for the rest of the war. In late 1944 Nos. 435 and 436 squadrons were stationed in Burma and India flying C-47 Dakotas and performing air drop missions and casualty retrieval. They did this until the end of the war and by the end of the war the 435 and 436 squadrons flew a combined total of over 60,000 hours and delivered close to 60,000 tons of cargo.
An excellent read on this topic is For Your Tomorrow: Canadians and the Burma Campaign 1941-1945 by Robert Farquharson
There was also members of the Veteran's Guards being used to transport mules in Asia, but I don't know enough to state anything on it.
In Canada, the Battle of the Atlantic is often discussed, but the sheer magnitude and importance of the campaign is grossly overlooked. This is an extremely long and in depth topic, so I'll stick to stats and numbers for now. As is often cited, the Battle of the Atlantic was the longest campaign of the war, stretching the entire length of the Second World War. At the beginning of the war, the Royal Canadian Navy consisted only of 3500 sailors, regular and reserve, as well 6 vessels, and by the end of the war Canada had the third largest Navy in the world (Behind the United States and Britain). By the end of the second world war, the RCN assisted with the passage of 25,000 merchant ships and 25 million tons of cargo to Great Britain. The RCN is also credited with the sinking of 31 U-Boats. This was at the cost of 1,700 sailor, and 14 ships. The Canadian Merchant Navy suffered even heavier with a loss of 1 in 10 of the 12,000 who served.
I got my numbers from Canadian War Museum Website, but some good readings on the topic are Atlantic by Simon Webster, Battle of the Atlantic by Marc Milner, and No Higher Purpose by W. Douglas.
Simply put the industrial strength of the United States swung the war decisively in the Allies favor. Here's a chart showing the warmaking capabilities of each nation based off industrial strength in 1937. As you can see, the US had almost as much warmaking strength as every other major Allied and Axis power combined. Even prior to the US entry to the war, US supplies via Lend Lease to the British and Soviets vastly supplemented their warmaking capabilities, including supplies and weapons as well as logistical equipment.