Excellent points brought forward. Here are some counter ideas.
@ Zuikaku
1) AA - I have not seen the AI effectively using this. As long as I have a couple of squadrons of the latest interceptors, I have been able to quickly mop up enemy air forces. And even so, the damage done by aircraft is small enough that you can ignore for the most part. Instead of building 10-20% of your land force with AA, just make more tanks/infantry and increase the speed of your advance.
The only valid argument I would accept for AA is that of Aragos, that there are some AA which are also pretty good defenders. Even so I would only build a modicum of these, but I don't. 
2) Anti tank - The static/soft anti tank is the only one you should ever consider building and only if your enemy is loaded with hard units. The hard anti tank units are very expensive versions of tanks with a slightly higher attack value against hard units, which tanks already have plenty off. The main problem is that this unit is to specialized. Aside from defending against enemy hard units it doesn't do much.
3) Destroyers are a valid point, the combo sub/bb can be easily replaced with dd/bb which is just as valid. In my recent game I did just that. This combo sub/bb or dd/bb has the purpose of defining 2 main roles for naval forces. First as an anti navy force, which is effectively occupied by sub or dd which have a good cost/time value compared to their anti ship capabilities. Second as an anti land force, where the BB is king. One critical advantage of dd is its air defense value. But I have not seen the AI making effective use of air force as an anti naval force.
4) This one is also @Aragos. Mech are one of the worst military choices in game. Let's see why:
- hard unit
- high supply/petrol consumption
- very high cost
- very high manpower usage
- generally lower combat values then tanks (across the board)
Why would you build a mech which is a hard unit (tank category) when you can just build a tank that is cheaper, stronger, takes fewer men and does the exact same thing.
@Aragos
You only need transports if you are going to land on islands which have no ports. That most likely means the piece of land is useless. Otherwise you can just use merchant marine (auto generated transports) and land through enemy ports. You will take significantly more casualties but you don't have to worry about managing transports. I would build transports only for:
- allowing the AI to automatically invade territories with no other means of access (he doesn't use merchant marine to do so, but he also doesn't really invade if you do give him the transports)
- so I can give supply support to my units (I do build a few civilian large cargo for exactly this purpose)
Proposal rehash
Once again the purpose of my proposal is not to give the optimum solution for combat. Intelligent use of all unit types will certainly lead to a better bang for the buck, but that requires minute micro management. I am a father with 2 kids, I don't get that kind of time.
I build my land force in somewhat the following composition:
50% infantry (no mobile or anything, special infantry or equivalent)
25% tanks
10% soft artillery
10% hard/mobile artillery
5% supply trucks
Infantry serves the purpose to defend important positions and do a slow advance towards the enemy. They have very good cost/power ratios cost little maintenance, consume no petrol and few supplies.
Tanks are the offensive branch. The AI doesn't normally use infantry to assault concentrated enemy positions and prefers to use tanks to do so. These are used to advance my conquest.
Soft artillery is used as a defensive measure for important positions.
Mobile artillery is used to bombard heavily defended positions.
Supply trucks for maintaining the advance.
I also construct supply depots as I advance into poorly supplied territory to maintain the advance.
For airforce, I keep a few interceptors to counter any enemy strike force and just a couple of tactical bombers when I encounter a heavily fortified position.
For navy, I just let the AI manage, but generally the big bulk is sub/dd and a few BB for land support.
How does this work in practice. Full initiative and minister control for all units I intend to use. I stay zoomed out at country level. Every couple of days I shuffle chunks of troops around. Troops slowly drift from critical areas to easy to conquer places and this regular refocusing helps keep the pressure on critical places.
Every now and again (especially with large countries such as Russia) the AI refuses to attack enemy positions and instead conquers out of supply territory. That's where it is a little annoying as you not only do you have to reshuffle troops every couple of days but also move them close enough to some critical enemy locations.
I generally focus my advance on large enemy supply centers, but in most cases I don't need to do to much management.
