For a folder, I suggest a Zeiss Ikon Ikonta or Super Ikonta,, in 6mm x 6cm or 6cm x 9cm, post 1930 or so, or, a FSU Iskra, which has Zeiss Glass, taken by the Soviets as part of their WWII reparations (they packed the entire factory up and moved it to the Soviet Union.
Both cameras produce nice images, though the Iskra is a pit delicate because of a very thin brass film transport gear.
The post 1930 Ikontas include coated lenses and are really a pleasure to use, with tight, strong standards.
The Iskra would take some looking, as there were few made, relative to a Fed, etc. and many had ugly holes cut into the backs by pro-photographers, and I was very lucky to find the one I did have, in that other than stiff grease lube, it was in great condition.
One more medium format, format and camera to consider would be a 4cm x 4cm (127) Primo Jr. aka Sawyer's Mark IV.
It has a great lens and viewfinder, both sharp and bright, much smaller than a 6x6, can fit in an overcoat pocket and a joy to use.
It is a great street camera as it almost looks like a toy and attracts less scrutiny from people too busy to pay attention on to the world around them
There is, I believe some 4cm x 4cm roll films being made, but you can buy a film slitter on Ebay and cut down 120 rolls,and trim back the ends of the paper, however, that will lead to some fogged film edges, so to keep this at a minimum, carry a small changing bag and do your cut-downs in the darkroom, with a green safe light, on only to trim ends and use black plastic 120 roll holders.
Used Bay 1 filters, Series holders and new threads to bay 1 adapters as well as, plastic lens hoods are easy enough to find.
Rolllieflex and Yashica both made "baby TLRs" but they are lesser cameras, in my and others opinions But I would be happy to have them in my 4x4 bag.