Every big city, needs and surely has a symbol. The megacities have even more symbols.
How must be a symbol of a city? It has to be interesting, historically tight to the city, impressive, easy to describe and, above all, it must have a story!
A symbol of a city can be a building, a statue, a monument, or even a square, or maybe a street. etc. I can give you some examples:
-the symbol is a statue: Miss Liberty for New York
-the symbol is a square: Red Square in Moscow, El Zocalo in Mexico City.
-the symbol is a street: Las Ramblas in Barcelona, Champs Elysees in Paris
The symbol must be easy reproductible and in lying in a crowded visited place. It must be magnetic and attractive, and lasting. Sometymes, the symbols, even destroyed, remain in the public memory (The Twin Towers from New York). It is a must in a city to find and visit its symbols, it is a part of your job as travelers!