Inca - 17
Korea - 13
Maya - 12
Poland - 14
I could just keep upvoting the underdog, but what the heck, I can't help but enjoy Korea. One of my favourite games was on a TSL Earth map as them. There, you could REALLY take advantage of their UUs: the Hwacha basically was what took China, and the Turtle Ships... oh god, the Turtle Ships. As Korea, that significant tech lead really pays off at the Turtle Ships. Sure, you can't go explore, but you are by and large the strongest coastal power. In that game I easily wiped out Japan and Siam using almost strictly turtle ships, and then began my march on the rest of the globe. Another nice thing about the Turtle Ship is that it improves the oddball upgrade line of Caravel -> Ironclad by just saying, "Here, don't worry about upgrading to Ironclad. These are strong enough already." And, to be honest, doesn't everyone want an Ironclad at the start of the Renaissance?
Poland and I can never seem to get along, and I think it may just be that they don't have much exciting gameplay - at least as far as I have gotten. It's why I like the Inca so much too - different strategies are fun! But Poland is basically Aztecs with production swapped for growth and a more unexciting UU and UA. Sure, power and all, but fun? Not as much.