Guild Wars 2, according to an NCsoft census that was translated on Reddit. The two-year old fantasy MMO has eclipsed 3.8 million Chinese users, which indicates far faster growth than the game has exhibited in the rest of the world. The title launched in August of 2012 and topped the 3.5 million sales mark a year later. China's 3.8 million sales happened in just two months.
GamesIndustry.biz posits that GW2's Chinese success is an indication of a lucrative and untapped market for Western developers as well as evidence that buy-to-play business models can succeed in an area dominated by F2P.
[Update: ArenaNet has disavowed these numbers as of July 11th. "We'd like to clarify recent news of GW2 China sales: it's inaccurate, from a Chinese fansite, and they could be estimating characters," the studio said via Twitter. Thanks to Martin for the tip.]