Earlier I mentioned Renown is important. If you want to rule Calradia, or help your faction of choice dominate it, you need an army. You cant have a large army without Leadership and Renown. Every 25 renown allots one extra soldier while every 1 Leadership allows for 5 (8 if you count the Charisma required), along with a percentage increase in morale and reduction in wages which gets quite important when your troops are promoted and your army size increases.
Again, you'll want to fight Sea Raiders on your own until your renown is high as the only way to keep soldiers you hire other than in your party is to garrison them in a city or castle, which you cannot own without conquering one and starting your own faction (and consequently, a war) or being awarded one by a King (usually after conquering one).
Fighting on the battlefield is dominated by calvary, archers, and heavy infantry. Vaegirs have the best archers, but they're slow on the map, terrible in a melee, and not very effective against shielded foes.
Its for these reasons that Nordic Archers and Rhodok Crossbowmen are the better choice. Nord Archers handle themselves quite well in melee and Rhodok Crossbowmen are better than most mid-tier infantry. Both ranged units can last very long in a siege without running out of ammunition, and the Rhodoks have the added advantage of killing in only one or two shots.
Nordic Archers may not be as good, but they're probably one of the most cost effective units in the game and very easy to train.
Nord Huscarls are hands down, the best infantry in the game. However, some swear by the Rhodoks as their infantry can stop a calvary charge. While this is mostly true, the absolute best way to stop a calvary charge is to force them to climb up a hill. Huscarls also have a very high chance of having a throwing weapon, capable of killing any soldier in a single hit.
On an open field, the lances wielded by Swadian Knights are unmatched (although Sarranids and Khergits are good in their own rights), and man for man, a group of Knights will make quick work of any similarly sized group of infantry, including Rhodoks. This is important to note as fighting the Sarranids happens almost exclusively on open desert, and their powerful archers and calvary can quickly overpower an unprepared army (even on easier difficulties).
I don't recommend acquiring Khergits as they are terrible in siege warfare nor do I recommend mercenaries for they are very expensive and typically are much less powerful for the denars you must spend for upkeep (with the exception of the extremely hard to acquire Sword Sisters).
If all you want to do is rule the open fields, answering to no one and being a free-brother like those deserter scum then by all means hire the Khergits. Their horse archers will literally run circles around the enemy.
If you do get to the point of having a massive army, Vaegirs are the cheapest and their highest tier units, while not as powerful as other factions, are very good if you outnumber the enemy.
Here, I have an ideal position. My Rhodok Crossbowmen are able to shoot over my Nord Huscarls. When the enemy finally gets close, an additional volley from my Huscarls ensures a quick victory.