I will say in general from the get go discussion about how "The Russians won WW2" are generally and unfortunately Eurocentric. While certainly much of the developed world was more immediately impacted by the war in Europe, and thus much of Reddit's user base, it should always be questioned.
It ignores the millions of Americans, Kiwis, Aussies, Brits, Canadians, French, Dutch, Tagalog, Chinese, Vietnamese, Polynesians, Koreans, Indian and others involved in the war against Japan. The killing started well before stormclouds darkened in Europe and continued after Hitler was dead and Berlin rubble. And was waged across half the surface of the earth and resulted in a similar amount of human lives destroyed, without a systemic industrialized genocide mind you.
To talk about all of WW2 when you really only mean the war against Nazi Germany(and maybe Italy and the other Axis Powers), it diminishes the entire other half of the global tragedy of the war.
Human losses is a huge part of the reason people say that the Soviet Union (of which Russians were only one part, though it was a large part) made the largest contribution to winning WWII. That is because, out of the roughly 60 million people who died during WWII, 27-28 million of them were Soviet citizens. 8-8.5 million of those were Soviet soldiers. The rest were civilians. One of the key arguments behind the Soviets-won-the-war argument is that the war would have ended before Western allies such as Canada, the Kiwis and the USA had time to enter it, if the USSR had not held out through 1941-1942. Yes, we should keep in mind the role of fighting in Asia when considering this. And no, we do not have to agree 100% with the statement that the Soviet Union contributed 'more' than the US. But in terms of loss of life, it isn't even in the same order of magnitude. So the contribution of the United States and other western militaries was clearly focused in a different way. I think it is fair to generalize by saying that the USA contributed to the war in a ver significant economic way. The Lend-Lease program was very important, including to the Soviets. The British made key discoveries in the field of intelligence. They cracked the enigma machine, and that played a role in the later stages of the war particularly. All these things matter. And there is no good, easy metric for weighing an intelligence discovery against human life, or the investment of $1 million dollars worth of tanks against human life. So I think it's ultimately an ahistorical question to ask who contributed more/the most, if we are not going to specify parameters. But anyone who wants to downplay the role of the Soviet Union in WWII should stop and think about the 27-28 million lives lost by that country during the fight against Nazi Germany. That is nearly half the total lives lost in WWII. 20 million of those were civilians. These people bore the brunt of German brutality for years (as did Poland). Deaths are as high as they are because of the incredibly low value the German occupation forces and front-line military placed on Soviet lives. So let's not downplay the role of other allies, but everyone ought to take a moment to appreciate how much ordinary Soviet citizens suffered and sacrificed. Canadians, Kiwis and Americans experienced nothing close to the loss of life and societal disruption that Soviet society did.
From /u/marisacoulter
So let's not downplay the role of other allies
I agree. While it's fine to speculate on the details of who-did-what, I find that answers to this question are almost always politically charged: either to diminish the efforts of the Soviets or the western allies. The very best way to approach it is as a team effort in which all parties contributed significantly.
Only a fool would maintain that the Soviet Union did not do the majority of the killing and dying, especially prior to 1944. North Africa and the Mediterranean Theater as a whole has been rightly called a sideshow, in that the forces committed were vastly smaller than on the eastern front and did not decisively impact the outcome of the war. However, it is easy to underestimate the degree to which the pressure exerted by the western allies aided the Soviet war effort, especially in the last critical year of the war.
The North African campaign accounted for more than 400,000 Axis casualties and caused irreplaceable materiel losses, especially to the Italian navy. The subsequent invasion of Sicily and Italy knocked Italy out of the war and added another 400,000 to 600,000 German casualties, in addition to the 1,000,000 Germans who surrendered there at the end of the war - men who otherwise would have been available to fight in other theaters.
By late spring-summer 1944, when Operation Bagration smashed the German Army Group Center and crippled the German ability to meaningfully resist, only about 60% of the German Army remained in the east. The remainder was split between defending Italy, France, and Norway and fighting partisans in Yugoslavia. Due to the allied bomber offensive, the best part of the Luftwaffe had been withdrawn to Germany and France, and would be largely used up in defending against this threat by the end of the year. The Normandy Campaign, while often overstated by westerners, was an unmitigated disaster for German arms, made all the worse because it occurred near simultaneously with Operation Bagration. The bulk of Army Group B - 38 divisions in all - was destroyed or crippled. More than a third of Germany's elite mechanized divisions was wiped out, permanently crippling the German capability for offensive warfare (the Ardennes Offensive was only possible by stripping units from all other sectors, and even then German forces were surprisingly weak and basically foredoomed to failure).
In summation, the sacrifices of the Soviet Union cannot be overstated. They were the decisive factor in the defeat of Nazi Germany; without their continued resistance, it's difficult to see just how the war could have been won. However, I find it equally difficult to imagine how the war would have gone without the western allies exerting such considerable pressure on the German flank. At the very least, the Soviets would have had to fight millions more men and thousands more tanks and airplanes on their way to Berlin.
Others have provided some good answers, but I'd also direct you to this old response I wrote on this topic.