To do this type of photography (the street) all cameras are good
but it is undeniable that smaller and less visible the better.
SLRs are good, you can change lenses and use the zoom to frame you bring the machine in front of an eye but they are big, bulky and recognizable.
The top of the street would be a rangefinder, leica type or Fuji digital talking about and some more opportunities if we think of the film.
Today have appeared on the market a new category of machinery the mirrorless, that have interchangeable lenses are small and offer a good quality.
However, all the brands are good, nikon, canon, pentax, among them today is not much difference at amateur level.