For each campaign, which unit could you absolutely not live without? The one unit that gets all the work done in your campaign. The MVP of the faction.
Level 1 garrison infantry are necessary for all factions, so you don't have to pick militia as campaign MVP unless they are exceptional (ahem, Sword Quendi). Same with Generals; of course Generals are necessary to the game, but are they really your work horse unit in battle? For some factions they might be, but not for others.
Here are my picks! Feel free to make a list for MOS or Vanilla, I am still learning MOS so my list is for Vanilla 3.2.
Dwarves - Dwarven Warriors
The dwarven middle infantry are insanely powerful. Normally shieldwall is reserved for Tier 4 infantry, but these Tier 2 units have it, along with 16 defense and armor piercing. Being Tier 2, they are extremely common through the whole campaign.
Dale - Dale Cavalry / Yeomen
Rhun will send nonstop waves of Balcloth Tribesmen as well as other ranged units. The Dale light cavalry roster is completely critical to their campaign, especially since their weak infantry won't be getting much work done alone before turn 42+. Dale is absolutely a hammer+anvil cavalry faction.
Mordor - Nazgul / Black Numenorians
In the early game, Nazgul really win all the field battles. I hesitate to use Generals for this list, but for Mordor it's really true. Press your fodder infantry into the enemy then rout them with Nazgul charges and screams.
Silvan Elves - Sentinels of the Woodland Realm
As a faction with no real cavalry, the Silvans have a pretty unique army composition of archer lines that can cut down stack after stack from Mordor. When you see a battle that is 3000 vs 20 dead, it's usually Silvans vs Mordor.
Orcs of the Misty Mountains - Snaga Skirmisher, Cave Troll, and Warg Riders are all critical
I tried to decide their MVP unit between Snaga Skirmisher, Cave Troll, or Warg Rider but they really need all 3 of those units to defeat the superior Silvans and Dwarves.
Free Peoples of Eriador - Merchants
Free Peoples are a pretty balanced faction and you can choose the archer strategy or cavalry hammer+anvil and make both methods work. Merchants are extremely powerful for their availability level in the game.
Gondor - The Unique Generals
Gondor is an extremely balanced faction with great infantry. I think they could get through the campaign without one particular type of infantry though. What really makes the campaign interesting is the unique generals. Faramir and the other archer general harrying Mordor stacks in Ithilien and then holding the river crossing. Boromir's citadel guard holding Osgiliath and then blockading Minas Morgul. Imrahil and his Swan Knights raiding the Harad coast. Gondor gives a series of unique generals with Tier3/Tier4 bodyguards which are critical to the early-mid game.
All right I won't do them all. What do you guys think?