If the Allies had continued to fight to Soviet Union after the defeat of Germany, who would have won?
This is one of those perennially never-completed arguments, so I though that I'd have a little fun and give my answer to this impossible question.
First, we have to examine the idea of winning. Winning is where one nation achieves its military/political goal, thus you must rate success by each country in accordance to its own goals rather than victory through an arbitrary measure, such as crossing the finish line.
We know what happened in real life. The Allied powers thought about continuing the war into Russia, but on consideration, decided that pursuing that option was not desirable. Likewise, Russia made the same calculation and decided similarly. Both sides would have preferred to see a more total victory, but even Russia saw its goal as unattainable.
I believe that the political powers of WW2 were correct in their assessment. If the war had continued in Europe, nobody would have won. Both sides would have failed to achieve their military and political goals. Given this conclusion, to continue the war would have been an unnecessary human slaughter. This didn't make the end of WW2 any less a mess, but it does inform us that the mess was preferable to a military solution.
But, what if the Red Army and the US Army had fought? In the short term, my money would be on the Red Army due to sheer numbers, and in the medium term, on the allies due to Russia's over-extended logistics and economy. In the long term, both sides would have settled on a peace. Many dead, little gained.