Well the obvious choice is a press camera! They are actually very quiet and you have a rangefinder. How about a graphic? Put on a rollfilm back and call it medium format. There are fast lenses available.

If you want something smaller and more agile, how about a konica hexar AF... stealthy, and with an all-purpose semiwide (35mm) lens. A contax g2 might also be nice and can make use of several great lenses, though it is louder.