Korea is still my favorite. Its UA+Rationalism (with some wonders like GL, Porcelain, Hanging Gardens, Pisa) gives crazy amount of scientific boosts. Not to mention the AI's change -- compared to G&K, the AIs are much likely to be friendly, thus making the RA much easier in BNW.
Also I feel like Freedom policies are made just for Korea -- all these specialist & great tile benefits.
Spoiler :
My first Deity victory with Korea
Korea has my vote too, I liked them a lot in G&K but in BNW they've gotten even better. Their UA works very well with the historical landmarks resolution from the world congress, especially with the freedom tenet that buffs great tiles (although it does suck you can't get 20+ science academies now). The science penalty for wide empires also makes tall science setups more effective which Korea was already killer at.