Inexpensive, fast, and able to hold its own against most other Land units, Tanks are a cornerstone unit in any army.
Tanks are one of the most important units in the game, with the early and mid game phases typically being decided by who can make better use of their Tanks.
The Tank offers a good mix of mobility, firepower, and defense, making for a very versatile unit. Tanks have little to fear from Infantry and Recon units, taking minimal damage from them and doing high damage in return. Tanks are also capable of attacking Artillery units from beyond their range, making them good units for combating exposed Artillery.
Tanks are evenly matched with Mechs and other Tanks, so getting the first strike is critical against these units. Caution should be used when fighting Mechs with Tanks, however; Mechs on heavy Terrain can still do moderate damage on counterattack, despite being less than half the cost.
B-Copters are the counter to the Tank, and should be avoided when possible unless friendly Anti-Airs or B-Copters are supporting the Tank. Despite their resilience to small arms fire, Indirect Combat units also deal high damage to any Tanks left in their range, so keep enemy Indirect ranges in mind when positioning Tanks.
As the game progresses, Tanks will become less critical to build, but are still vital units to any army, being good escort and support units for higher tier units. Tanks are quite efficient at dealing damage to Anti-Airs and Indirect units, opening up new target opportunities for friendly B-Copters and Land units, respectively.
The most readily available counter to a Tank is a B-Copter. B-Copters deal high damage to Tanks, and take little damage in return, all while being able to keep up with the Tank's high movement very easily.
In maps with dense terrain, Mechs and Indirect Combat units can also fight against Tanks reasonably well while being less constrained by Terrain features. Mechs will typically be reactionary units in this case, however, as their slow movement leads them to being outmaneuvered by Tanks fairly easily. Positioning Mechs near more valuable units will deter Tanks from attacking those units.
Indirect combat units are very effective at fighting Tanks as well, but will need support and protection from other units to prevent enemy Tanks from moving in and crippling them. Infantry do this job very well, as despite the Tank's strength against them, they cannot wipe out Full-HP Infantry units with one attack. The Infantry will still be left at very low HP, however, so a proper formation is needed to make sure only the Tank is capable of hitting the Infantry, lest two units take out the Infantry and expose your Indirect.
As the game progresses, higher tier units will be available, all of which having little to worry about from Tanks. Bombers and Stealths can both attack Tanks with impunity, and Md. Tanks/Neotanks/Mega Tanks can all deal very high damage to Tanks while taking little to no damage from them.