As for modded civs, Prussia is insane if you get orders. Depending on settings, you can have a very good chance to get them (raging barbs with god of war or one of their unique events). You can get range or logisitcs on every ranged unit in medieval. You get very tough infantry. And all of that before your UUs even arrive. All these bonuses apply right when you build units, no need to level them up by fighting. Together with a massive production bonus for land units, you can even throw away many units at will.
I was positively surprised by playing America. Their ability to buy enemy tiles and put a citadel there, combined with early pioneer forward settling, makes combat very easy. You can set up forts and roads all along and you can get your army deep into former enemy lands before you even declare. And you have early economic bonuses that help getting a bigger army before the fight starts. This helps a lot with chokepoints and difficult terrain, which can be the biggest obstacle to conquest. You can't attack a city with sige units because there is forest in the line of fire? Just take over those tiles and chop.
Those are not strictly military bonuses though.