That depends on what you want to experience. Beijing is ideal for imperial history and the Great Wall. Xi'an is known for the Terracotta Warriors and ancient Chinese history. Shanghai offers a more modern urban experience, while Chengdu is popular for pandas, Sichuan food and access to western China's landscapes. For nature, travelers often consider Guilin, Zhangjiajie, Sichuan or Yunnan. A good first itinerary usually combines two or three different sides of China rather than trying to see everything.