I really don't like these hypotheticals so I'm going to be as non-committal as possible.
If you're talking about war between the Allies and the Soviet Union directly or shortly after WW2 such as in
Operation Unthinkable, then I think Russia would have the upper hand for a long time. Contrary to belief, Russia was never the land of endless human resources (~160 million pre-war to 120 million for the US), especially not compared to China today. They certainly did not have more people than the UK and the US combined, especially after losing around 27 million people, including at least 9 million soldiers during the war. Compare that to less than 1 million deaths combined for the US and the UK. However, you combine Soviet armor with battle hardened PPSh-equipped soldiers and tactics that allowed them to crush Germany and I can easily see the Russians pushing the Americans and British accross the Rhine in no time, especially since the Russians are the only ones actually sharing that continent. After that, it's a bit of a toss-up. Again, the Soviet Union is likely far more exhausted from the war than the Allies. Although, if they could repeat the 1940 situation and successfully capture France, I think they'd have Europe in the bag. I don't think Overlord is going to work twice, not without someone distracting Russia in the East.
If you're asking who's been doing the best job in the second half of the last century, well, there's not really much to compare here. How am I supposed to know who would win between France and Russia based solely on first-world countries beating down on third-world ones? All we can say is United-States, Israel, UK > Vietnam (though they didn't "win" the war I don't think you can fault their K/L ratio) and Middle-East, Middle-East again, Argentina respectively. Considering how average Russia has done in Chechnya and Georgia, I don't think they're doing so well.
Who's the toughest single G20 country today? Again, too much conjecture. First of all, no nukes allowed, only an idiot would nuke someone he shares a planet with. In a Fox-only, Final Destination, no items match, I guess it's either the US or China. US combines a huge military with better soldier preservation shown in WWII. China has a bigger potential military, but we have no idea what they're capable of. For all we know they could have trained each as a Navy SEAL, or they could be a jobber-militia. One thing's for sure, neither is capable of invading the other across the sea. Any other country would need to be massively better than the US for its size to be able to take them on.
Finally, who does better per-capita? Again, there's no way of knowing which country has the best training, equipment, doctrine and leadership. We just don't have the data that can only be seen in war. I'd personally rather we not find out.