Khergits: All units are horsemen starting from a very early branch. They have no infantry line, and their archers are mounted. The lancers aren't too spectacular, but they have decent armor and make good shock cavalry. They are great when commanded correctly (micro-managed in battles), but are overall very weak. I don't think you're looking for this faction.
Sarranids: Amazing cavalry that are on par with Swadian Knights, a decent archer line, sub-par infantry line. The archers may not be as strong as the Vaegir marksmen or as accurate as the Nord Veteran Archers, but they make up for it with a very fast fire rate. Their infantry is terrible though. If you're looking for a cavalry based faction with the addition of archers, you might as well take this one.
Rhodoks: Best crossbow units in the game, highly efficient for attacking and defending sieges. Lacks cavalry, with most of their units sub-par until they hit the final tier. Their units aren't the best in the field, but a competent player would command their troops rather than just spam "Charge" like the cavalry based factions. A solid nation, with good map placement and perfect for conquering all of Calradia.
Swadians: The best cavalry in the game, but a rather infantry and archer line makes the faction a one-trick pony (pun intended). There isn't much to do with this faction besides spam knights, which is terrible for sieges. The good news is that your little band of knights can generally take on any challenger in open field. The infamous King Harlaus resides here with terrible map location, which is a large annoyance to most players. I suggest this nation if you're a fan of all cavalry, all the time.
Nords: Best infantry in the game, decent archers. Has a weakness of being unable to field any cavalry, but the huscarls more than make up for the lack of them. Huscarls are equipped with throwing axes and solid shields, being able to soak up the advancing archer's arrows then throw their axes in the approaching cavalry's face. Great in sieges, as the huscarls can soak up many arrows and bolts with their shields while plowing through the line with their high damage. The road to huscarls is long though; many recruits will die before they ever reach it. Another solid nation, with the armies comprised of infantry and archers.
Vaegirs: Great archers, a decent cavalry line, but sub-par infantry. Their marksmen wield the powerful war-bow, trading fire rate for incredibly high damage. They can break shields incredibly fast, which is always a boon. They lack shields, however, and are vulnerable to enemy archers. I tend to use the archers the most Their cavalry, while not as spectacular as the Swadians or Sarranids, are more tank-like, and comprise a decent portion of the army. Their infantry isn't all that good, but they are mainly used as shock troops to absorb the impact of enemy cavalry and archers while the marksmen take their tole. A great faction with a good starting location.
I rather like the Vaegirs for the fact that they are the jack-of-all-trades, meaning more strategy and less spamming of cavalry. I feel as if the Vaegirs get the least attention in the game, due to their rather average troop lines. However, it's this very averageness that I feel at home with, as they are the only faction known for always giving a good performance.