Xi’an is one of the oldest.（city）in China.

Xi’an is one of the oldest.（city）in China.
Xi’an is one of the oldest.（city）in China.

Xi’an is one of the oldest.（city）in China.
cities,因为古老的城市有很多

cities

cities
one of+最高级+名词复数

cities
西安中国最古老的城市之一