Okay, you all got me curious! Closest I could find was Heihe, clocking in at 8,800 km. The closest "major" city is probably Harbin at around 9,200 km. Shanghai comes in at about 10,600 km, Hangzhou at 10,800, and Xiamen at 11,400. Turns out because of curvature the shortest path from basically anywhere in China is going to be over the North Pacific to Tijuana. Thanks for the fun little excursion y'all :)