Pole-arm

Since pole-arm Mastery is the skill both profitable for charging cavalry as well as pikemen, the answer is - Templars and Swiss halberdiers.

I like Swiss guys more. They are experts in charge counter and that is the main point of defence infantry.

Since I don't have neither of them (and quite doubtful I will ever have) it is largely a theoretical point of view, but my arguments are as follows:

1. The fact that all units will counterattack looks good on paper. But who are these units? Melee or charge cavalry, melee Infantry? To be honest they are quite weak at counterattacking. Archers? They will counterattack anyway.

2. Since Swiss halberdiers reduce charge probability, they have relatively high chance to survive if attacked by charge cavalry, while Modao will certainly be destroyed, no matter how powerful their counterattack is. Probably such a trade-off is useful in PvP battles, when we are willing to sacrifice a legion to destroy Richard and Co. But normally I would like to keep my legions intact and to heal them when it is needed (medical tents, war drums).
Modao is a perfect unit for Dingfang, but any other general with Modao will be dead within seconds when trying to stop cavalry charge. And to stop cavalry charges is the primary role of Defence Infantry.

3. Modao are better suited than Swiss guys for attacks, but they are not as good as melee Infantry in this department. They have too many offensive skills for a defence Infantry unit and too many defensive skills for an attacking unit, imo.

Like I said, it is only theoretical assumptions. And we need help of yuanzhong to test them in real battles. But my guts say me that Swiss halberdiers are more steadfast and better at counterattacks.

As f2p
Heavy phalanx and noble cavalry as they are in high lvl unlike the others.

The units for pole arm mastery depends on you. If you have templars and swiss in high lvl use them.

With Su Dingfang active skill, they literally 1 shot in counterdefense higher leveled Templars, Noble Knights and most cavalry. Infantry gets heavily damaged also, the only doing winning a trade on him on attack are archers.

But y, its his unique troop and this skill just makes them shine even more than they do normally, as receiving almost no damage can make the counterattack superdeadly. On my other infantry I normally have either Varangian with horse archers, Varangian with melee Cavalry or Varangian with Heavy Phalanx (as I have them lvl 15 vs Modao's lvl 6), depending on the second best skill of the General.

I do have swiss unlocked, but because they're only level 3, no general has any of them. Anyways, of the F2P friendly troops, Heavy Phalanx is the best troop.

At lvl 12 Swiss halberdiers should aquire lvl 3 Halberd skill. This will reduce the probability of charge by 40%. It is quite significant, I think, since charge is the most damaging ability in the game.