Honestly, I believe it is best to play ultra conservatively with Russia and England and see how the dice line up. If one Axis power ends up in an exceptionally poor position after their first round, America can then make the choice between KJF or KGF.
However, here is a run down for you:
Kill Germany First:
Easiest strategy. Everyone is clustered around Germany so coordination is easier. This is also much more fault tolerant then going after Japan. However, Germany also has the most assets and the best capability of getting assets, not to mention all of their territories are land locked (comparatively speaking of course) resulting in much easier defense of those territories. Exceptions are Norway, Algeria and Libya, but those 5 IPC can be made up for in other places if needed (and if not, 35 IPC is still a lot of money.)
Kill Japan First:
Japan’s by far the weakest power on the board. Their territories are scattered all over the ocean and they start the game facing two moderately powerful fleets. (UK: 2 Transports, Submarine, Fighter, Carrier, Destroyer in SZ 30 and America’s Battleship, Transport, Destroyer, Submarine, 3 Fighters and Carrier in the Pacific (SZ 52, 55, 20, Hawaii and W. USA.)) However, they also have the benefit of being scattered all over the place making it harder to take their territories (but easier to hold once they are taken.)
If facing a large build up of forces in Buryatia, Sinkiang + Industrial Complex and India + Complex, Japan can find them selves in serious trouble fast; but Germany will have much free’er reign in Europe and Africa.