I like to keep rhodok sharpshooters on hand since they are resistant to damage, pack serious punch and are deadly accurate. And then some kind of infantry, sometimes rhodoks, sometimes huscarls, and sometimes I do without infantry and keep only knights with my rhodok sharpshooters. They are of course extremely mighty on the field, but they also do very well attacking sieges, as well as defending.
Only in castle garrisons do I keep bow archers, never on the field, as they are way too lightly armored. Defending sieges they are useful and they get ammo replenishments from time to time.
Overall I feel that the Swadians have the best troops, as their knights are mighty, their Sargeants are plenty good infantry and their sharpshooters are useful archers in the right situation. Whereas nords don't have any cavalry, Sarranid guards are meh, khergits are just all-around weak, Vaegirs are a bit weak in the infantry and archery, and of course although rhodoks have great infantry and archery, they have no cavalry. So if you are going to go with one kingdom, get the swadians. IMO.