Not asking which one you think is most powerful, but rather which one you've had the most fun playing as. For me it has to be Arabia. They are not the strongest civ, but the madrassa is my favorite building in the game. Get 6 envoys with a couple of scientific city states, slot in the natural philosophy policy card (doubles campus adj.) and you will be generating a disgusting amount of science and faith per turn. I won a science victory with them the other week, was generating over 450+ science and 450+ faith a turn and I only had 5 cities. Highly recommend every try a turtle science victory with Arabia.