That depends upon your preferences.
Most typical is 35mm. Gives a good chunk of depth of field if used at f/8.
The founder of street photography, Henri Cartier-Bresson, used 50mm. Requires to learn to focus quickly, unless you have enough light for crazy apertures. Of course, today we are in the age of autofocus. With a good system, you can relax in this respect. But back then it was manual focus only.
Another famous street photographer, William Klein, used 28mm. Even more depth of field than 35mm, but you need to get really close to your subjects.
Of course 21mm, 24mm and 85mm (or 75mm / 90mm on Leica) are possible as well.