It all comes down to the player's tactics, preference, troop availability and terrain.
Khergit Veteran Horse Archers and Lancers are deadly on open flat plains, especially when used in hit and run tactics.
Swadian Knights are great for charging at infantry and archer lines on flat terrain. Just make sure they don't engage the enemy if greatly outnumbered. Attack weak flanks, run off for a bit to break unit cohesion. Charge in again. Rinse and repeat.
Vaegir and Norse Archers are great on hillsides because they can shoot for a longer distance and at a greater pace than crossbowmen. Norse Archers seem to be more competent at hand to hand, but it's been a while since I've played Vaegir so I've forgotten.
Rhodok and Swadian Crossbowmen are ideal castle defenders. Especially when grouped together in great numbers. I remember trying to take over Praven THREE times with greater numbers than the defenders with Nords, only to fail because of the tenacity of the crossbowmen.
Rhodok Spearmen, Rhodok Sergeants, Swadian Infantry and Swadian Sergeants are great defensive infantry units. They are slow and because of their pace, don't present any good unit cohesion on the field. Put them in an infantry line waiting for enemy light cavalry to charge and watch the slaughter.
The Nords are special because they have fast moving infantry and are armed with short ranged missile weapons such as javelins and throwing axes. Their weakness is attacking enemies waiting on hills because when they charge in, they tend to throw a few missiles at the enemy which exposes them to enemy archer fire. Either tell them to Hold Fire or attempt to flank them. They are the best infantry on flat terrain however, but their weaker armor compared to the higher tier Swadians and Rhodoks don't make them very good defensive units.
These are my own personal opinions, so I could be wrong. I'm just saying that a good general (and player!) shouldn't just attack the enemy anywhere, with anything. If your Company is mainly cavalry, draw your enemy into flat terrain. If you're leading a mostly infantry Company being pursued by a mostly cavalry foe, either wait in a forest or even better a forested hill or mountain side. Use the terrain, use the battle menu. There is no such thing as "THE BEST UNIT", it depends on how you use them.