Assyria is crazy O.O All you need are two siege towers early game and you're on your way to awesometown. Sure, you COULD keep those cities. But alternately, you could simply take them, hold on to them through the war, steal a bunch of techs, and then give the cities back for a tidy sum. Or spread them amongst others, all the same. Thing is, if you steal 3 to 6 techs, you already got such a crazy headstart its insane. Their UB isnt particulary great, sure, but those Siege Towers + Stealing techs = Awesome.
I'm sadly going to have to say Indonesia. They're too map-specific compared to the others. While the Zulu are one-dimensional, they do what they want to do very well. Morocco is a bit underwhelming compared to what I expected but they do well. Portugal is stronger then I thought. Venice is fun to toy around with, Shoshone can go out of hand real quickly, Poland, as boring as its bonuses are, has a very flexible game particulary due to those simple bonuses, and Brazil...well, they're a one-trick pony too. Just a very powerful one that needs some time to survive the early game though.
France and Arabia are both still really solid.