After playing a bit the game, you quickly realize that two of the most critical days, when it comes to research, are days 8 and 16, due to the many good options they provide and the possibility for mistake they allow. Day 24 is equally overloaded, but since many games are already practically decided at that point I will not deal with it at all.

It is clear that the more things you have researched the better. The question here is “what do I research first, with so many options?” Of course, the specific answer you will give depends on your personal style (I am a planes fan, or I am a land units fan etc.) – this text will not try to change your style, but just to give a flexible guideline which you can accommodate according to your taste.

For me, the most important rule I follow is:

If I am engaged in a dangerous battle, the target is to select upgrades that neutralize the opponent’s advantage or give me a tactical advantage based mainly on the existing units. Otherwise, I select upgrades that enable me to put quickly useful new units in the field.

Let’s start with Day 8 as an example: at this day the first special units become available, but there are also the extremely important upgrades on LT, air units, arty, AT and AA – not to mention the new armor units (MT and TD).

Following the rule mentioned, if I am engaged in a battle I will pick first an upgrade that helps my defense/attack, like the LT, arty, interceptor, or tac bombers (depending on the situation of the battle and the army of my opponent).
On the other hand, if I am not engaged in a battle with a human player (and I don’t foresee a battle for the next 1-2 days), I will pick one of the new units for one of the slots – MT or Commandos, depending on your style of play – and only use the second one for strengthening my existing army.

The general rules for this day are:

• If you foresee a battle with a human player who has strong air force, aim for Interceptors Lvl2 and AA Lvl2. Interceptors Lvl2 not only are much stronger, but their bigger range will also save some resources while building your airbase network.
  
• Unless there is a very specific need, usually RR Guns should never be a high priority at this point (also, it is NEVER a huge priority unless you can have some Infra Lvl3 in order to build them). Same applies to naval units – in case you have some naval research still pending (but, again, this can change if you face already a naval battle). Still, it must be noted that Cruisers Lvl2 have a bigger range so they can efficiently “hit and run” enemy fleets of DD/CC (although it is not likely to happen in day 8, as it is too early, you have to keep this in mind).
  
• If you plan to go for Commandos asap, remember to upgrade the Barracks of the capital to Lvl2. That is a mistake I have painfully made far too often. Barracks Lvl2 take a considerable time to build, so it is a pity to have to wait while you have prioritized Commando research.
  
• Although Tank Destroyers are excellent tank killers, if your opponent doesn’t seem to have Infra Lvl2 then MT can perform the same defensive role but with extended attacking capabilities against Infantry units. Check the newspaper and your spies before deciding on your strategy.
  
• Interceptors Lvl2 have the same range as Rocket fighters. If you build an efficient airbase network for this range, it will later also serve perfectly for them as well (should you choose to go for them).
  

NOTE: I ignore here paratroopers, that may or may not be useful in a specific scenario. There are many reasons for that (including my lack of experience on them). Mainly, I doubt they should be made a priority in Day 8 (who has airbase Lvl3 in the capital by that time?) but I am not sure for later days.

Day 16 is even more clustered. There are some extremely important new units (like SP Arty and SP AA) plus a “vital” upgrade on the normal artillery. Following the same rule, my strategy is usually as follows.

• If I am already at war with a player, or if I have even the slightest fear that I might be in war soon, my first upgrade is Arty Lvl3. To be caught with insufficient range is equivalent to death against an active opponent with artillery force.
  
• At this day planes become extremely powerful, so they take up first priority as well. Again, whether I will upgrade first the tac bombers or the interceptors depends on the specific situation.
  
• I try to put SP Arty or SP AA as soon as possible in the field, depending on the army of the opponent.
  
• I only upgrade as fast as possible the armor weapon that gives me a specific advantage. If I need speed it is the LT; if I need firepower it is the MT.
  
• Rocket Lvl2 is usually not a very high priority, especially in such a clustered day. Additionally, one can “postpone” slightly this research, build a mass of rockets Lvl1 (that can be built in ANY airbase), and then upgrade them en masse.
  

The coalition touch

The previous was more about single play. When coalitions are involved, and especially if they are at war, the country that is not in the “front” should try to assist the actively engaged player, by providing fast support in the areas that he will be (by necessity) slow or even will neglect them completely. For example, in Day 8, the active player might need to upgrade his LT (that are fighting) while the supporting countries can go for MT or even RR Guns, weapons that will really add strength to the front line.

Now it is Days 4 and 8, exactly.

Unless of course you mean that now strategy is obsolete in the game. Which is unfortunately very true, after the latest changes.