I'm still sticking to Luoyang, which was the capital of Eastern Zhou. Anyang is closest to Ye, the Shang dynasty capital (first archaeologically documented Chinese dynasty), but for gameplay reasons Anyang is rather wasteful. Chang'an was simply too far west to be defensible, even though the Western Han dynasty and later dynasties tried it there again.
And as far as the Qin being the first to unite China and had the capital in Xianyang (south of Chang'an), one should read Wikipedia on the Qin in the Spring and Autumn period:
"Qin was in a rather backward condition compared to its rapidly transforming neighbours to the east. The demography of Qin consisted of a large proportion of Sinicized semi-barbarian people, believed to be descended from the Rong tribe of the steppes. This was a main factor that caused a distinct unease and discrimination amongst the other states in China toward Qin at the time."
Only later did their power grow to enable it to unite China, and the Qin dynasty did not last long, whereas the Han dynasty did (and again had Luoyang as the capital in the Eastern Han).
As for game play, even if some mountains were changed to hills, Xi'an cannot grow big until windmills (see Hattusas). Whereas Luoyang can be a monster site with lots of rivered plains and only 2 unusable tiles (mountains).