I voted China. These guys are the absolute gods at dominating the world. The momentum from the Middle Ages GA and building all those riders which then upgrade to cavalry just keeps stacking up and rolling on until there is no one left but China. [I didn't mean to play the Chinese last month, it was a joint game with a friend which got me back to playing them]
I also had a game, my first game, with the Celts. Monarch level as usual, continents, absolutely wiped the floor with them too. I should really have voted for the Celts but I left my heart in Beijing, China soon after my first game with them.