Formal:
* A metropolis - emphasizes size and importance
* An urban center - focuses on the central role of the city
* A municipality - highlights the city's administration
* A conglomeration - emphasizes the density and mixture of people and things
Informal:
* A hive - suggests a bustling, active community
* A jungle - evokes the city's chaotic and competitive nature
* A melting pot - highlights the diversity of cultures and people
* A hub - emphasizes its importance as a center of activity
Ultimately, the best collective noun for a city will depend on the specific situation and the message you want to convey.