It depends on the doctrine, what your doctrine has buffs in you should produce them. That does not mean you don't have to produce anything else, you still have to produce inf. tanks, planes, and arty.

It depends on your doctrine of course but in general this is a good strategy:

The most important unit is artillery (against armour) and rocket artillery (against unarmoured). It's the only unit
that can deal manage from a range without taking any damage itself. When you put a meat shield in front of your artillery to protect them against enemy attack they will last you all game long. Put a couple anti air in the stack with the artillery as they are very vounerable against air.

My perfect stack is: mot/mech infantry, medium tank, SP anti air, SP artillery, SP rocket artillery. The mot/mech infantry and medium tank will stop anything that wants to attack your artillery, the SP anti air will protect your artillery and the combi or SP artillery and SP rocket artillery can take out anything what the enemy throws at you. These units are also pretty fast and it's very hard for your enemy to find an answer against this combination.

The Mot/mech infantry can leave the stack to grab empty provinces. In support you will have tactical bombers (against armoured) and attack bombers (against armoured). They will deal with any single fast units that want to slip trough your lines and will provide air support when in big battles. Of course your bombers need to be protected with interceptors against enemy planes.

My unit set is:
Mot inf
medium tank
SP arty
tacs, attack bombers, fighters 2:1:1
submarine

I found this to work for all doctrines since it fits my play style, on big maps and often doing quick rushes. All sorts of foot troops are too slow to keep up then.

My stack is

Land
Anti infantry: motorized infantry, fast, good attack, excelent vision

Anti light armor: light tanks, fast, good against inf, excelent against light armor

Anti heavy armor: heavy tanks, excelent against heavy armor but really slow
Alternative: AT unit, really cheap but slow, it SHREDS tanks

Anti air: depends on the doctrina

Support: mostly rocket artillery, standard artillery is worth it only when your enemy has heavy armor


Air

Every class of aircraft

Naval bombers depends if i Will be fighting in the sea, sometimes is not worth it to research

Strategic bombers are sometimes good in the endgame to destroy factories to slow down unit production, or to simply be annoying, i just use 2-4 units


Sea

Anti subs: obviusly destroyers, cheap, fast, really strong, they defend my carriers

Anti ships: Carriers, 10 naval bombers can attack you from 400km and your enemy won't see it coming, plus, they hit really hard

Subs: sometimes i use them for scouting or to attack fleets with good AA defense, in this case i don't use destroyers

Cruisers: i just use them if my enemy has good naval bombers

At least 50% of lvl 1 units should be reasearched. So you can have quick respond to any threat.

Max 33% units should be reasearched till lvl3, max 20% unit till lvl5 and max 4 kinds of units above lvl5.

All units are worth to be produced. It depend only on situation on battlefield and your own strategy or strategy of your opponents, if you are underdog.

As last warrior said you should always leave some units low level to allow quick production. My advice is to have conventional infantry and artillery as low level.

If you are axis, make sure to use Motorized infantry and medium tanks.

Make sure to check the doctrine info before choosing what to produce.

+1 to check your doctrine, adjust production accordingly, in addition to adjusting for starting location which you probably know already being a 1.0 veteran.

+1 to leaving some units at level 1 because with every upgrade units get more expensive AND slower to build. Once you get to level 2-3, it becomes impossible to build that unit in a front-line city. You need to improve the factory, and you can't do that until you improve construction speed by building propaganda offices, and the whole thing becomes a giant PITA.

Bonus hint: Research level 1 militia and LEAVER THEM at level 1. It's the only unit that can build quickly in a recently taken city that starts with 25% morale. It takes ~1 hour to build the barracks, and then another ~1 hour for the first militia. And now you have a unit that can go out and grab more land. Anything else you try will take DAYS, so don't even bother.