Most to least difficult:
For new players:
1: Japan
2: Germany
3: UK
4: Italy
5: Russia
6: USA
7: ANZAC
For somewhat experienced players, or at least how I feel about it:
1: USA
2: UK
3: ANZAC
4: Russia
5: Germany
6: Japan
7: Italy
China is just an extension of the unified allied resistance in the Pacific.