There is no single best first turn build. It depends on the overall strategy.
I've put some notes below for each nation, based on the 1942 online version of the game used in ranked play.
If the allies decide to "Kill Germany First" (KGF), then a standard buy for soviet r1 is 4 infantry for caucusus and 2 tanks in russia. The 2 tanks in russia can reach karelia, and can ensure Germany cannot put a large stack of 9 infantry and 4 tanks on r1 without risking a big loss.
However, if the allies decide to "Kill Japan First" (KJF), then swapping the 2 tanks for 3 artillery or 4 infantry may be wiser, as the goal is to delay losing moscow for as long as possible while US are attacking Japan's Navy
Also, the two tank buy makes less sense if russia does the "west russia jam" (ie all 19 ground attack west russia and ignore ukraine), as this already stops Germany from holding karelia in r1
I think it is always sub optimal to build planes on soviet round 1, as the battles can easily go badly, and you need a lot of ground if that happens.
For the germany r1 purchase, there are a few options, and it depends on what russia attacked and how well the battles went. I think it is a good idea for germany to delay a uk fleet for a few rounds, and having 5 fighters in north western Europe and 1 bomber in Germany or Italy typically will deter the uk player.
If the German bomber in Ukraine is still alive, then maybe buying ground is the best option such as 11 infantry and 2 artillery.
For the British, a good "all round" purchase is 2 infantry, 1 artillery for india and 2 fighters for England. This works well for either a kgf or kjf.
However, again it may be better to swap the two fighters for a destroyer and carrier - depending on the threat that germany has placed on sea zone 7.
But note - one risk of building a UK fleet on round 1 is that it delays the 2 fighters in England from getting to West Russia and India/Egypt. Being able to defend and counter attack Egypt and/or Trans-Jordan in the first 3 rounds is important, and 2 fighters are very useful for this. If you build a fleet you give up units here.
For japan, I think there are at least 6 "good" options, depending on what happened on the previous turns.
- 3 transports, 3 infantry
- 3 transports, 1 infantry, 1 tank
- 2 transports, 2 infantry, 1 fighter
- 2 transports, 1 fighter, 1 submarine
- 2 transports, 2 infantry, 1 artillery, 1 tank
- 1 submarine, 1 aircraft carrier, 1 fighter
Options 1-4 are all good choices when the only attack was sz61, leaving Japan with only 1 transport at the start of their turn.
Option 1 leaves 7 land + aa gun on Japan with 4 transports. Maximum ground pressure through China.
Option 2 is a "nerfed" version of the option 5 India attack I describe in more detail below
Option 3 has 1 less infantry and 1 aa gun stays on the Island, buy you get an extra fighter.
Option 4 is a "hedge" against a kjf, and gives japan a sub that can be useful in later rounds to harass unguarded US transports in sz11 and sz10 in a kgf. It still allows Japan to send 6 ground from an island on turn 2.
Option 5 is likely the optimal move when neither sz61 nor sz37 were attacked. It allows the most amount of ground units possible that can reach india by turn 3. ie you can land 9 troops in Yunnan on and 4 in Anwhei on j1 (leave the tank on Japan), then you move those 9 to Burma, along with sending the two transports you bought in r1 with 4 more ground, for a total of 13 in Burma. Then you use the other two transports to move 2 tanks and 2 infantry to Yunnan, and you move 2 infantry from Anwhei to Kwantung.
Then on round 3 you can send the 13 ground in Burma, 2 tanks from Yunnan can blitz through, you can transport 2 infantry each from Yunnan, Kwantung, and East Indies. This is a total of 21 ground units. If japan purchases 2 bombers on turn 2 and keeps all their starting planes (6 fighters + 1 bomber), this is 30 attackers on india + 3 bombards from ships.
Now, normally this isn't actually possible, as the moves require battles where units could be lost - eg it is more normal to only be able to get 10 or 11 units to Burma. Also, units may be attacked - eg the UK may have enough units in India on r3 to attack the Burma stack. It is also possible for the allies to use a ship to block East Indies from being picked up.
Additionally, it is more typical for this "J3 timing attack" to attack sz53 (pearl harbour) on r1, Which usually results in the loss of 1 fighter and 1 cruiser from the india attack, and possibly an extra fighter and the bomber. The reason to do this - is that it is possible for the US to get 3 fighters to india by turn 2. So trading a slightly weaker attack for a much weaker defense makes it more likely for japan to win.
It tends to work really well if Germany has a good start in Egypt and/or Trans-Jordan, as there isn't enough units to be able to defend both.
I think option 6 is the best buy if the UK did the sz37 attack and won, as it is a pretty clear signal that allies are coming for Japan first - if uk bought strong units in India and/or Russia moved 5 units into Buryatia are other signs. It replenishes two of the units lost, and Japan only needs two transports in a kjf, as the goal is to stay alive and delay the US naval dominance for as long as possible.
For the US, I think there are two pretty good standard buys
- 2 transports, 1 destroyer, 1 aircraft carrier, 2 infantry - for going after germany - deploy in sz11
- 2 submarines, 2 destroyers, 1 aircraft carrier - for going after japan - deploy in sz56
The variations on these two basic purchases depend on how the other nations went. For example, in option 1...if Germany has 5 or less planes and no subs left, or japan did not attack sz53, then swapping the destroyer for another transport in option 1 might be a good idea. Or if Germany built a carrier, then swapping 1 transport for a submarine.
I haven't needed to vary my US1 purchase if I've decided to go after Japan. I find this purchase is usually good. But this is likely because I normally look for a "setup" to do a kjf - eg russia had strong opening, sz37 went well, or Japan had bad luck on its turn (eg failed to take Anwhei with 3 or more left, left transports vulnerable, lost a few planes, etc). So, when I tend to do a kjf, the board is in my favour already. So the purchase doesn't matter so much.
I think purchases that try to put things in both Pacific and Atlantic are sub optimal - the US should "pick an ocean" and stick to, spending 90% or more of their income on the side you picked.