Varangian Guards, Swiss Armored Pikemen, Janissary Heavy Infantry are all among the best infantry, but I don't think there's any single unit that makes a faction a 'must play'. You've got to consider the whole unit roster and when you're starting (early/high/late).

- Overall, I think SAPs are the toughest. They can excel against infantry and cavalry, but they're a Late era unit and require a Fortress, County Militia & Master Armorer - unless you capture a relatively unscathed Switzerland, yer gonna spend years and years on buildings. By then the game is nearly over. Same thing with JHI - they have lengthy build requirements.

- VGs have manageable build requirements, but take 2 years per unit to train (4 years when using huge unit size). Viking Huscarles are available in Denmark/Sweden/Norway in Early, cheaper, low build requirements, 1 year training, and very near VGs in effectiveness.

- All the late/catholic heavy cav are like rolling thunder, but they all require extensive infrastructure. Which nearly always leads to - by the time you can build them, you don't really need them.

I guess what I'm saying in a long-winded manner ( ) is IMO there is no best bar none unit. That's part of the beauty of MTW, there's no single uber unit.

edit: to continue with Oaty's thread, if u are looking for the easiest beginner faction:
1 - English/Early; easily defendable island, trade network fairly easy to get going. Clansmen(scotland), gallowglasses(ireland) and with VI the various viking units (huscarles,carls, landsmenn) by jumping on the Danes - which have easily defended provs too.

2 - Byzantines/Early: VGs, ByzInf, Katanks - all excel against Early era units. Plus Byz are blessed with uber rulers who pump out uber princes to lead your uber armies.