I have two approaches - a 45mm fixed rangefinder and a 24mm lens on my Nikon SLR. The rangefinder is super-sharp and quiet, while the 24mm lets me shoot without even looking through the viewfinder. The latter is probably an approach that will get me criticized by some, but my guiding philosophy in photography is "whatever works."

So any fixed focal length from 24mm to 50mm is good. If you have interchangeable lenses, there's no reason not to gradually build up an arsenal.