The knob wind Retina II is a good camera and prone to fewer problems because it doesn't have the shutter cocking rack of the later cameras. The Retina II requires you to tension the shutter separately.
However, as long as you find one in good shape, these can be on your list:
- Retina IIa: The lever-wind camera is small and has a great f/2.0 lens, although single-coated, I believe.
- Retina IIc/IIC: Has the slightly slower f/2.8 Xenon/Heligon and lacks a meter. I sort of like this model.
- Retina IIIc/IIIC: Integrated uncoupled selenium meter with the excellent f/2.0 Xenon/Heligon. The large "C" models have frame lines for the auxiliary telephoto and wide angle lenses and larger viewfinders.
- Retina IIS: Often overlooked, this is a smallish non-folding rangefinder with a very nice f/2.8 Xenar lens and coupled selenium meter.
If you're good at guessing distances, you could go for one of earlier folding cameras. You should know that the older cameras offered many different lenses and shutter combos. And you should also expect that all of these cameras might require some level of service from routine to total overhauls.