I'm voting for Swadia.
Swadian cavalry are so powerful and they're always the first recruits I'll grind. Whenever I'm at war, I'll usually target their king/queen first just to show my dominance (O_O) by defeating their leader during the first stage of war, however whenever Swadia declares war upon me, I'll avoid their leader because there are always too many casualties for me regardless if I win the battle or not because our forces are pretty much equal.
Swadia are often bullied based on experience which I find extremely unusual. Another honourable mention would be the Nords because of their infantry but they don't stand a chance against caverly.