I could never do that (unless I really had to).
For me it would have to be a 50mm lens.
Probably, my M and a 50.
Main Cams: Leica, Sony, Nikon, Fuji, Olympus, Pentax, Panasonic, Canon
Main Lenses: Leica, Nikkor, Voigtländer, 7artisans, Meyer, Sigma, Pentax, Tamron, Rokkor etc.