The most prosperous is definitely Chengguan District.
And the busiest in Chengguan District is:
(1) Xiguan Cross (all kinds of snacks, night market is very hot)
(2) Dongfanghong Plaza
Lanzhou's several major districts: Chengguan District (the seat of the government), Qilihe District (the Muslim agglomeration area), Xigu District (the industrial pollution area), Anning District (the university district), Lanzhou New District - Qinwangchuan.