I think Germany is the hardest with Russia being a close second.
I think Germany is the hardest because even with a large bid, if you have one screw-up or bad roll it can really cost you, whereas if Russia has a bad roll it is a bit easier for the UK and US to go into full protection mode of Russia then it would be for Japan to help Germany.
Germany has a lot of first rd “must” attacks that a bid may not help, they have to kill the Brit fleet, take out the Gib BB, possibly kill the sub in the Med, possbily take out the UK trn off Ecan, and take out Egy, and that ignores Europe. Â Even with a PAfr bid that is a lot of battles that can possibly go wrong.
Russia worst case, can play extremely defensive early on and still be quite a powerful force mid game.
It is always cheaper and easier to defend, thus the pressure is always on the Axis to do something and shift the momentum early on so you can make your move sometime in rd 4-8, but that is easier said then done and any delay or set back can be costly.