If you want an army that can beat everything you either have to play on easy (like I do... yes, I'm ashamed) or field an army that picks the best from everyone.
There was a guy around here earlier, called Ludial, he's inactive for the moment but will come back. He claims that an army of Vaegir Marksmen can take down anything you throw at it, and he also claimed it was really usefull in sieges, when he assaulted a castle or city he would order his archers to stand ground and empty their quivers, then retreat and repeat.
If you play on easy you should have no problem wiping the enemy with an all cavalry party, I do that, I have Vaegir and Swadian knights, Khergit Lancers and Right now I have a heap of Mercenary Cavalry too, cause I needed to pump my size up quick.