I have used the Leica III a lot on the street and have enjoyed using several different lenses. This includes the Nikkor 3.5cm/2.5, the Color Skopar 35/2.5 and the old standby, the Elmar 50/3.5. They all seem to work very well but I would recommend going with a later, coated Elmar if you decide to go that direction. Obviously both the Nikkor and the Color Skopar are coated. Of them all the Color Skopar is the newest and is also the most resistant to flare.
I have also recently begun using a Kiev 4 along with a Color Skopar 35/2.5 or Jupiter 8, and find it to be just as nice on the street. If I find one at the right price I may give the Jupiter 12 a try as well. I also own a Snapshot Skopar 25mm lens but am not as comfortable with that field of view as I am with the 35mm.
You gotta love a fast lens;
It is almost as good as a fast horse!