Having a fondness for folders, I now own or have owned many of the cameras discussed here. The Retina IIa was my "first". It's rangefinder is adequate to focus the lens, but the viewfinder is of the dim, squinty class common to most 35mm cameras of the 1950s. Generally, the Zeiss Ikotna/Super Ikonta viewfinders lack by comparison, and their front cell focus is optically inferior at closer distances. They are also over priced as a user. I've owned two Mamiya 6 folders. They tend to suffer from coating failures in the lenses, giving low contrast or clouded images. They are very heavy for what they are, but have decent viewfinders. A recently serviced one at a good price is clearly a choice, but stay away from the ones with the Olympus lenses. The Konica Pearl III has a very good Tessar copy lens and good, simple mechanics, however if you have the price, a Konica Pearl IV in very good shape is a completely different camera (except for the lens/shutter), and it is a joy to use. You are going to pay a "collectible" surcharge for the Pearl IV. The Voigtlander Super Isolette may be the model of the perfect MF folder, but having never use one, I'll leave it there. The lesser Isolette models suffer from triplet optics, which as a common user should be avoided unless your budget is small. The Isolette III was offered with the superb Tessar copy lens/shutter on the Super Isolette, but they are extremely rare and expensive for what you otherwise get. All of the foregoing use 4-element Tessar copy lenses, which are more than adequate for the format and its modern uses. However, if you want the best optics in a relatively modern MF folder, the Fuji GS645 is in a class by itself. It has well deserved issues over its original bellows, so if you shop one, either have the seller produce proof that the bellows has been replaced or deduct about $125 from the price of a good one to pay for your inevitable repair. I'll skip the 1950s era 6x9 cm folders. The modern GF670 Fuji (6x7/6x6 formats) or its Cosina/Voigtlander equal is out there, somewhat overpriced. It seems to be showing some problems as it ages, which would exclude it from my consideration. If you want the bigger format and your wallet will take the chance.....