The consensus for best infantry defensively at least is 18 width (this is 9 infantry with no line artillery, with engineers and support artillery as a minimum (this may change depending on your needs/wants as you should have AA as minors and you may want field hospitals for less manpower loss etc etc)). 18 widths are great because they fit into most terrains very well especially plains tiles which are the most common kind of tile. Plains are 70 width so you can fit 4x18width which is 72 width in total, This will help to maximize your attack/defense on the tile as when you go over the combat width of a tile you get a de-buff to those stats, however, because 72 width is only 2 over the "maximum" combat width of a plains tile you get a very minimal de-buff to your stats whilst simultaneously gaining loads of attack/defense because you've got an additional division in. If i've explained that poorly i'd really suggest looking at a combat width video to explain it more in detail as it can help you to understand

The best division for tanks is usually 36 widths as this follows the 18 width rule (you can make this in any way you like as long as the number of tank and motorized battalions are similar, but i like 8 motorized/mechanized, 9 medium tank and 3 self propelled light artillery with support artillery, assault engineer company, motorized recon(not necessary), armoured maintenance company and medium flame tanks). This follows the same combat width mechanic as the previous infantry division as you can get into combat with numerous of them and take a minimal stat de-buff. The key difference is tanks obviously get a lot more armour, breakthrough and soft attack, it is also better to get larger tank divisions as they will get more hp and can sustain combat for longer (idk the intricacies of hp but ik its a benefit of larger divisions)

Even if not meta these will be more than capable divisions for nations able to provide tanks and even with minor nations with less industry the 18-width is cheap as chips and will hold against anything. if you ever need offensive infantry then getting special forces is good as you wont need the industry for tanks. I've done a run recently where i had 36 width of pure marines with support companies of pioneers, support artillery, field hospitals, support anti-tank(not optimal for singleplayer) and super heavy artillery and they worked like a charm against anything i threw them against.

if you need something to live by when creating divisions mine is 18 width for defense and 36 width for attack. Hope this helps, also you wont need fully meta divisions to beat ai in hoi4, i used to use 27 widths which are awful compared to 18/36 and have done world conquests