A best strategy is always dependent on a situation and the counter strategy your opponent is using.
personally I think a German strategy of sealion or strangling britain through convoy is the most effective.
I also think A USA strategy of 100% pacific is very effective but not if germany goes sealion.
Italy should always focus on control of the med. africa and the middle east (in alpha +2) if you are playing oob I think the best thing italy can do is can-open for Germany. Russia strategy is pretty straight forward.
Japan is extremely complicated as there is so much to be done, it really depends on the situation on the board. India smash, Kill america first, kill australia first (less of an option in alpha +2), all out China/Russia etc.
Anzac needs to be as annoying as possible, usually done with lots of subs.
An Indian fleet always seems to be the best for Uk Pac. at least early in the game but only if there will be at least a moderate sized american fleet.
China needs to be annoying by spreading its forces out and keeping the fighter alive.
Uk London is tricky, I have experimented with building planes each turn and it seems to be pretty effective but assuming there is no sealion threat the best thing to o is probably build ground units in S. Africa and a fleet in canada.