I would have to say putting all the countries in order of hardest to easiest I think it would look like this:
Russia, b/c its got both Axis countires at eather side
Germany, falls into the same situation as Russia for the most part, but it has a much higher income
UK, I picked them next b/c it would take some time to muster up a force and get them into place to retake or hold most of their territories
Japan would have to be next, I put them in fourth b/c their empire is much more consolidated then the UK, and they have the biggest advantage of being able to see a pending invasion at least 3 turns in advance.
Lastly the USA what can I say for the most part in the real WWII the US was only as involved as they wanted to be. They have the highest income and have to fight the least to hold it, for example if Japan took Midway and Hawaii the us is only out 1 IPC. And like Japan ( but not to the degree in my opinion) they can see an invasion looming on the horizon.
Thats my two cents,
-LT04