Without a doubt, this is my favorite camera. The Zeiss Ikon Contax IIa is the one camera I will keep, regardless of what happens to everything else I own.
To me, it represents everything that a camera should be: refined, quiet, excellent ergonomics and great lenses. Zeiss Ikon was on to something great when they created the Contax IIa.
This was a refined postwar version of the Contax II. It uses a narrower rangefinder base, but still is very wide and offers very accurate focusing.
Although the Contax I, II and IIa share the same lens mount specifications, the three are very different from each other. The Contax IIa was an all-new camera, built from the ground up.
In my experience, the Contax IIa black dial is a quieter camera than the Contax IIa color dial. Both are excellent cameras, although I tend to use the black dial most. The color dial allows you to connect directly to an electronic (or bulb) flash without the need for the intermediate cable. Otherwise, they are functionally identical, except for the comment about the shutter noise.
The camera takes an assortment of lenses from the classic 21mm f/4.5 Biogon to an excellent and much-underrated 135mm f/4.0 Sonnar. It also takes even longer lenses, which require you to use a large reflex housing.
My favorite combination for an outing is the Contax IIa with the 35mm f/2.8 Biogon and prewar 85mm f/2.0 Sonnar. Some days, I'll just take the two lenses and leave the 50mm f/1.5 Sonnar at home. Whichever glass I choose, I can be assured of excellent results.