This question is ultimately for a lens to pair with my anamorphic lenses, but it applies to non-anamorphic shooters as well so I have posted it outside of the anamorphic forum.
Looking at adapters, it appears the shortest flange distances (and thus, adapters) are C-mount, Contax, M39, and Pen F in roughly that order.
My only criteria is that the lens would need to be at the very least 45mm (maximum 85mm), and with a non-rotating front -- you'll see that this eliminates most "pancake" lenses.
The M39 Industar 50 (50mm f/3.5) and Contax Zeiss Tessar 45mm f/2.8 look like good options, but I'd like to start a discussion and see if anyone knows of any other good options.