Vaegir Marksman are the backbone in any situation they get in. No pikes / long spears for anti-cav and their shields aren't very durable. Vaegir is a balanced faction that excels at offense, at a cost.
Also, swadian knights and xbows are both good enough as infantry that you don't really need many sergeants if any, they are just knights without a horse for the most part (but cheaper too). The great lance is just so long, only the Rhodok pikes can match the distance. An army of knights in vanilla Warband doesn't really have any weaknesses other than horse archers in equal or greater numbers. Just stick with Swadia and keep producing their mascot, the swadian knight, like 75% knights, 20% xbows and 5% infantry. In the end it just depends on how you want to play and what you want your army to be made of. Your main goal would be to just protect your archers, but they are workable. The cavalry will keep the other cavalry off your archers, but probably would be great shock cavalry against hordes of infantry like Swadia. Their infantry is worse than Nords and probably a little worse than Swadia's heavy infantry, but they work well enough as a distraction to protect your archers as they massacre the enemy. If you want to build a balanced, more realistic army I would go with Vaegir or Sarranid. Crossbows are better than bows in siege because they don't run out of ammo as fast as bows and they still pack a punch. Their cavalry can take care of any field battle while their heavy infantry will hold their own in a siege. Swadia would probably be the best if it wasn't in the center of the map at war with everyone. Khergit has horse archers and cheap units so you can build an army fast. Rhodok has best crossbowmen, but bad infantry and no cav.
I don't really like playing Nords because I rather have good ranged units or cav so I don't know much about their other units. Nords have best infantry, aka best siege unit, but are lacking elsewhere.
Swadia makes the best field army, but it is very expensive to upgrade to max and upkeep. Swadia has best heavy cav, aka best field unit, and decent infantry and crossbows. The have mediocre infantry and cav with the best archers. If you want to build a balanced army go with either Vaegir or Sarranid.