For heavy cavalry, Swadian Knights or Sarranid Mamelukes are about equal in open battle, but as of version 1.127 my favorite heavy cavalry are slaver chiefs. They are the new top of the manhunter troop tree. They are more fragile than knights or mamelukes, but they carry steel shields and heavy military hammers, making them excellent for taking prisoners and earning you ransoms. None of the three are really effective in sieges, either attacking or defending, since they lack ranged weapons. The best part about slaver chiefs is that they are neutral troops, so they can be used against any faction without morale loss.
For infantry, Nord huscarls are the alpha and omega, tough, well-armed and armored plus they throw axes. Most carry heavy shields so they can get in close and dish out some punishment. Rhodok sergeants are probably the next best infantry.
For archers, Vaegir marksmen are just that, marksmen with a high rate of fire. Rhodok sharpshooters are crossbowmen, so their rate of fire is slow, but their bolts really pack a wallop. They are excellent for siege defense, but in a field battle their slow rate of fire can become a liability.