I only have a few games under my belt but, like anything, it would depend on your style of play. I know I love getting a few armoured cars into an enemy's rear areas and watch them run amok while the enemy uses a ton of units to run them down, which could be of better use at the front. Saying that, if he/she has a well developed air infrastructure and air force, it would not cause much of a problem. I prefer speed of unit over hitting and staying power. Can't hit what you can't catch lol

I would say having medium tanks and armoured cars. So 2 reasons. Medium tanks are basically medium which takes the penalties of speed and firepower the same on both sides so they are equal and armoured cars are great scouts or great sight range

I also am a total noob, but my thinking is this:

Ignoring speed, just looking at cost and firepower, the heavyer tanks are very much better than their smaller counterparts. I think all 3 can have a place in your army.

The slow heavy tank seems very good to me for pushing an attack together with other slow stuff like artillery. Of course you'd want to add some AA to that.

The small tank's speed matches with mechanized infantry or motor infantry. Combined they have some fighting abilities both on the open fields and in cities. They cannot have AA or anti tank support with that speed, but they are awesomely fast to quickly take empty lands from the enemy, or quickly intercept stuff that sneaks behind your main forces. (i am currently playing whack a mole after i was surprised by the AI taking over the inactive players, i don't know if this is a very common situation, but the speed squad is awesome for this)

The medium tanks speed matches with mechanised AA, and tank hunter. You can of course add mech infantry to the mix as well. This would be your versatile force that has speed to move to where it's needed relatively quickly while being strong and versatile enough to handle all kind of situations.

I personally like to just constantly upgun myself as I tech up, so I often start by using light tanks, then medium tanks, then I use a combo of medium and heavy tanks to form a stalwart defensive line. Of course, I often play defensive, and see the value of armored cars and light tanks there, but I prefer firepower over speed.

I prefer to focus on producing SA artillery, and grow it into a large group before attacking the enemy from afar, it only has 3 levels + Elite to research and are cheap to produce and use little oil, 50 I think. The issues is they aren't available from the start, and need AA and AT to increase survivability.

Dont know if anyone has said this yet, but Light tank spam. Early game build infrastructure in all your industrial complex provinces and then start churning out light tanks. Armored cars cannot do effective damage against infantry, but light tanks demolish early game infantry and miltia. Later in the game, start diversifying more and building medium/large tanks. Tank destroyers are a no go for me since they are honestly not worth the cost. Heavy tanks are great for holding provinces and also absorbing damage. SP AA also great for moving stacks that cannot deal with slow regular AA. SP Artillery is way better than artillery but eventually you will start getting oil shortages so dont use too much of that.Overfall, light tanks are great since they are fast and mobile but can actually do good damage

For some reason, I always go with SP arti and SPAA as the core of my late-game army with a large airforce... nothing ever goes wrong with that simple combination. That said, I do think an addition of AC for sight range or LT for better melee damage doesn't hurt its speed and protects its weaknesses. I think MTs are pretty good, but I never have the research priority to get them, and it's been several months since I last used them. HTs, I avoided for a long time, but I'm sure they're excellent at holding key strategic points with enough support.

The Only support HTs ever really need is Infantry for urban environments, and regular AA. It has a high ant-infantry attack value, and an even higher one when defending against them. Its anti-tank value is a full couple of points above the MTs, and its defense value is again higher. All in all, it is great for spear-heading an attack and holding what it takes from counter-attack, with its only real weakness being speed (and Urban environments)

Urban environments are the real weakness of tanks, otherwise they are pretty much good counters to most things.