What's the scariest playable AI faction?

[econ21]

My son asked me this question and I could not answer him: what is the scariest faction in M2TW? Let's forget the Timurids and Mongols, as they are special cases and would probably romp home as poll winners. Also, the Aztecs and the Papacy.

I guess I am asking which faction provides the most formiddable opponent or, perhaps, which one would you worry most about if you found them bordering you or marching an army into your lands?

Playing England in the early game, I would choose Denmark. They sometimes march a full stack into the Netherlands and have a lot of decent melee infantry before I have any swords of my own.