Why do so many people think infantry is so good? Sure, they’re cheap fodder, and their importance cannot be understated, but they’re just not good at all for assaulting areas alone, and you probably want to mix in tank/art with them in trannies rather than load 2 inf per trannie.
If all you do is build infantry then you will never be able to advance. Their attack sucks, they’re not as flexible as planes/tanks. I think it’s a great idea to mix in art/tanks so you actually have the attack power to invade well. Massing infantry gives you good fodder and cheap defense, but if you look at the numbers, their offense really really sucks.
Unless you’re the king of rolling ones, you’ll find your mass infantry falling flat on their faces against defending infantry or mix of infantry/tank/fighter. I think it’s usually best to start with lots of infantry because they move slowly, then build tanks/fighters to catch up them and do a combined attack, whereas if you keep massing infantry you have to wait another turn for them all to meet before going in, which gives the enemy extra time to build infantry on his end for defense.
Also if you have an IC in an isolated area, you want to build the most quality units you can afford there because the mobilization limits are usually very hefty (like 2 or 3 units). While building infantry is obviously the cheapest and best defense, having 3 tanks there is better than 3 infantry if you can afford either one. Infantry is the best when your deployment limits are higher than the number of infantry you can build, but if you have spare cash it’s better to go for quality attacking/defending units, since 3 infantry still take the same number of hits as 3 tanks.
I think the most versatile unit is the fighter. Yeah, it’s fairly expensive, but it has great mobility, ability to attack sea and land, has best defensive value, and isn’t forced to move into a country just invaded (can’t even land there) which is usually weak after you get it. They don’t need transports to move around, and are comfortable with sinking large navies from afar and then helping you take land.
I wouldn’t recommend massing fighters purely, but sometimes they’re just the best thing you can build for nations like Germany/Japan who need to deal with navies and land. After you see a good germany/japan use their fighters to great effect you’ll see the use of fighters.