I was in Chinatown earlier tonight and had only one lens with me. If you are to take only one fixed-focal lens (non-zoom), which will it be? It is a personal choice but I like the 50 mm (25mm in m43). It is neither too wide or too tight and as such is great for street use. Because it is a bright f1.4, the lighted signboards provided sufficient illumination and speed was a manageable 1/60th of a second, even at ISO 200.
Only limitation is the narrow plane of focus and you have to decide which you want to keep in sharp focus. Here, I focused on the lady. Pana-Leica Summilux paired with the Olympus OMD make a perfect night street photography combo.