I'd say Republic of China, really because of the mobilization potential and the nature of warfare at the time. You can't win an attrition war with China after all.
Question, do you know what the actual situation in China was at this time?
Anyway:
On land, Germany. Industrial base, population, and the large standing army gave them the edge here by a long way.
At sea: United Kingdom. Again, industrial base and large standing force gives it to them.
Absolute: I'm not sure you can really compare the two really, as their militaries were geared toward different things. On land the UK couldn't beat Germany, but it wasn't meant to. The UK's allies were for that, with British support and troops fighting alongside them to overcome Germany's preexisting advantages. That means ultimately Germany can (and was) worn down by the overall allied powers on land even if none of them could individually beat it straight up on their own.
By contrast Germany's military was built to win a land war quickly, but as was shown by the blockade and war at sea was basically helpless in the face of the British fleet. So I think I'll give it to Britain, as her military was strong overall, and the relative weaknesses were made up for by the strength's of her allies. Germany by contrast had allies who did not offset its weakness at sea (amongst other things because there really wasn't an option).
In order I'd go:
United Kingdom
Germany
United States (yes it had a relatively weak military the geographic realities of North America as well as the vast potential of American mobilization offer enormous long-term advantages, while being able to thoroughly hold back short-term defeat)
France (which remember fought with much of its industrial base under occupation)