For medium tank, armor is the most important stats, get wield armor, sloped armor, and +9 or higher armor boost. Then you will need to give +9 or more to speed to recover speed. Use the best medium cannon. They have the best total soft attack + hard attack and piercing. The medium tank will provide the tank division with armor, piercing, breakthrough, and hardness. Try to keep reliabiltity at 80% or higher, but make sure your tank division have armor bonus vs enemy regular infantry.
For light tank in armor recon support, or supply a mix tank division, the armor will not be enough to give armor bonus so you use the cheapes armor and go for soft attack or breakthrough. Automatic cannon is cheap and good. If you are poor or need to produce tank fast then you start with cheapest option first but the same chassis. After that you can change production to more expensive version, but don't change chassis unless you need more armor, to keep production high.
If the light tank can give armor bonus, then build it like the medium tank above and use the best small cannon.