I think the best of the 126 cameras is the Rolleiflex SL 26. It offers match-needle metering, three lenses (actually three front elements), a Synchro-Compur shutter with speeds from 1/2-500 plus B, a rapid-return mirror and a very nice image for focusing. The battery powers only the meter.
Rollei also made the A26, a viewfinder camera with a push-pull operation, zone focusing, automatic exposure and a nice Sonnar lens.
The Zeiss Ikon Contaflex 126 has true lens interchangeability. The metering system is battery-powered shutter priority. It lacks a rapid-return mirror.
I haven't played enough with the Kodak Retina offering, but I recall that I was not impressed with it.
The difficulty is in finding a camera that works. The first Rollei SL 26 I bought didn't work, and the second stopped working until I was able to free a jammed gear. Its mechanism is very complex to service -- even more than a regular SLR because the same amount of gears are jammed into a space that is roughly half that of a normal SLR.
The Rollei A26 is a simpler camera, and these often work without too much difficulty. Generally, the companion flash that was sold with the camera either doesn't work or is missing the AC adapter that is required to charge the battery for the flash.
I've yet to encounter a Zeiss Ikon Contaflex that works properly.
Of the two, I prefer the Rolleiflex SL 26. I find it easier to use, easier to focus and easier to meter. It's much more straightforward in its operation.
Both Rolleis and the Contaflex use the 625 mercury cell for the meter. You can easily substitute a hearing-aid battery with a rubber "O" ring in its place.
I might also suggest a 120 camera (or even 127). You could get an inexpensive twin-lens reflex or perhaps a folding camera or even a Holga or Diana or one of the myriad other 120 cameras that offer the square format.
In 35mm, there is the prewar Zeiss Ikon Tenax and Tenax II (great camera, but expensive) and the Robot. The postwar Tenax (with a Tessar lens) or Taxona also could be on your list, although either can be expensive as well (roughly $100 or more).
I agree with you about the square format. It's terrific.
I have
reviews of both Rolleis on my site.