I can’t really decide. All are important. Good cautious but opportunistic China play can give Japan something to worry about. France can frustrate the Axis in various small ways early on if things go right for the Allies or the Axis makes a mistake/gets bad rolls. Anzac has some potential to sneak something in against Japan or at least garner some of its attention when it might be best focused elsewhere.
On the other hand, Italy has some potential to contribute to the Axis effort if it can get a little momentum. The Med makes for some interesting match ups. Of course, Italy can get bottled up with little income and various forces breathing down its neck…that’s not much fun. Italy has the most potential, but gets fewer turns to play…