Portugal and England are basically ezmode countries. England has the french hurdle to get over, that is, they need to lose their continental holdings, but once they succeed in getting their shit pushed in by France, their godmode is inevitable. Just seriously stay off the continent. Your manpower is better spent fucking indians, in the west and the east.
Portugal is both the easiest country and outperforms history as either the player or the AI, by a significant margin.
My favorite country recently has been Venice. Their potential to wreck shit in the mediterranean is just awesome.
I don't encourage playing as native americans, africans, or most east asians, though japanese daimyos and manchurian hordes can be cool. The game is very much designed around european expansion, not the resistance against it.