First off, exclusively use 20 and 40 width divisions for combat. 40 width is objectively superior to 20 width, and should be your end goal in any nation you play that has enough pony power to make enough divisions to cover your borders.
New Mareland, County of Griefwald, Socialist Republic of longsword, Nimbusia, the breakaways in the Equestrian Civil War, Wittenland, Penguinland, and maybe Firtree, Greneclyff, and Farbrook should remain 20 width, although frankly even with Greneclyff and Farbrook I tend to go 40 width. It’s acceptable to use 20 width in early wars even for larger nations like Olenia versus the Changelings or the Equestrian Civil War, but both nations should switch to 40 width thereafter. All major nations, plus Olenia, Stalliongrad, Skyfall, Dread League, and also the Riverlands nations that gain cores over the other nations (think of fascist Lake City and Communist River Republic) should eventually become 40 width. But basically, ask yourself “do I have enough ponypower and production to produce enough 40 width divisions to cover my current and anticipated borders?” If yes, then go 40 width, of no, then go 20 width.
Division templates are extremely important. A basic but extremely powerful template to use is the 20 width 7 infantry 2 artillery template. This template will beat most, though not all, enemy divisions in one on one combat if you have modern equipment. A better infantry division is the 40 width 11 infantry 6 artillery template. This infantry division has so much artillery in it that its model displays itself as an artillery division rather than an infantry division. It is probably my most produced division template, and I use it for defense, to fill the gaps in the line, and to kill enemy pockets. It is not the core of my attack.
The best division of the entire game is the 40 width medium tank division: 9 medium tanks, 8 motorized, and 2 Motorized Rocket Artillery or Medium self propelled gun. This division will melt away virtually any opposition, only really failing when its organization is very low or when it is outnumbered by more than 3 to 1. It’s as good of a defensive unit as it is an attack unit, allowing it to hold the line after making breakthroughs, and defend itself against enemy attacks without the need of infantry. It’s very fast, allowing it to encircle enemy forces easily.
Between these two templates I usually create a third division that’s kind of between them. Medium tanks are amazing, but they are insanely expensive. As such, I usually make a cheap but fast alternative. One such unit is the 40 width Motorized a Rocket Artillery Division: 11 motorized, 6 motorized rocket artillery. This unit has the coolest looking and sounding model of the entire game, with an amazing sound in the attack action. It’s not as efficient in breaking through enemy formations as the tank division, but it is fast
, quickly encircling enemies, and efficient at taking victory points. It does require many factories to produce, and it’s easy to set aside a few early. Unfortunately, the nations that benefit the most from this kind of unit - like Lake City, Olenia, Changeling lands, or Greneclyff - don’t have domestic supplies of rubber, which seriously cuts back the cheapness of this unit. Another option is the 40 width Light Tank division: 9 light tank, 8 motorized, 2 motorized rocket artillery or light self propelled gun. This unit is efficient at breaking through, and is very fast. Unfortunately it’s almost as expensive as a medium tank division, as is only cheaper by virtue of allowing factories for it to begin production earlier.
Those three templates will do pretty much everything you need, but special forces can be a thing. The best special forces unit in the entire game is the pony Pegasus division. It is fast
and can be paradropped. These units are best as pure 10 Pegasi 20 width divisions, with 20 pegasi 40 width divisions being even better. These things are as fast and as powerful as rank divisions when armed with magical weapons. The next unit to be considered is the Changeling Jager, which is not as good as Pegasi. I tend to either neglect these because I am running out of Army experience make templates by this point, or make them into 14 Jager 4 motorized rocket artillery or medium self propelled gun templates. Deer nations get bonuses to mountaineers, so I make mountaineers if playing Olenia. I use the same 14 Mountainer 4 motorized rocket artillery or medium self propelled gun template as with Jagers, basically. I don’t play Griffins enough to know much about knights, but you can neither drop them from planes, nor are they faster than infantry, so there isn’t much reason to not mix regular artillery.
Finally, sometimes you want garrison divisions to stop naval invasions. The 8 width infantry templates that you get at the start of the game are frankly strong enough to stop the great majority of AI naval invasions.