In a medieval society, what government would be concidered most stable, with minimal infighting?

I've seen a similar question regarding governments in the modern era, but seeing as that the majority of history seemed to have been spent in a medieval sort of society, I was wondering: of all the governments, both practiced and hypothetical, which do you think would have been the most stable in a pre-gunpowder, slightly agricultural society? This could include theories of government proposed by philosophers, military leaders, eastern/western/african/american systems, etc. Personally, I was thinking perhaps some sort of theological monarchy, like the Chinese or Aztec empires, but I may be wrong. What's your thoughts?