For me rarity has always taken second place to affordable.
I've had some odd-bod cameras in my time but my collection, now reduced to about 15 from a 'high' of 60+ when I retired in 2012 and decided to sell off as many as I could to clear the house and keep my marriage intact, I would rate as entirely "what I could afford".
Overlooking my four Rolleiflex/Rolleicord TLRs which are wonderful machines but not all that rare, my most unusual (again a personal opinion) camera is my Leica iif, bought at an estate sale a few years ago, not cheap but at a reasonable price. I've added a few lenses and other Leitz bits and pieces as they came my way and I now have an enjoyable kit, one I can easily take with me out in the field without bursting the bottom of my backpack.
I've owned more than a dozen Rollei TLRs in my time, including a Tele and a Wide Angle model, both sold off at a time when they didn't fetch as good prices as they do now, and which I had kept. Again, not ultra rarities but seldom seen nowadays.
Of these Rolleis, my favorite 'shooter' is a Rolleicord Vb in user condition but mechanically functional, bought as a kit with a big box of bits and pieces, again from a deceased estate, for $95. I use it with a 16 exposure kit. The Zeiss Xenar lens on it produces amazingly good images.
Next on my list are two German folders, a Zeiss Nettar from ca 1950 and a VOigtlander Perkeo I. Again, not quite one of a kind cameras, but enjoyable to use and they fit into a coat pocket with a Weston Master, V one or two filters, a lens hood and some 120 film.
Another camera I seldom see out and about today is the Contax G1. At one time I had four of these but over the years I've disposed of three. Now have only one which has a somewhat unreliable rewind (at times it works and now and then it doesn't, so out with the black change bag (aka the darkroom T-shirt) and a bit of tugging to get the damn film out without ripping it. Last year I sent all my Contax Z lenses to consignment sale, the 45/2.0 went in a flash but the 21, 28, 35 and 90 all gathered dust on the shop shelves so I'm leaving them to my stepkids as part of my estate when I pop off. I use them now and then, my favorite is the 28/2.8 Biogon and the quality of the negatives (who can afford slide film or E6 processing these days?) never cease to impress.
The Contax G lenses, while made in Japan, are nothing short of superb, if with an unfortunately obscure mount which renders them useless for other cameras unless one invests in an expensive mount rebuild. I wonder if these will ever be collectables.
I envy those who luck into fantastically obscure cameras, especially so when these are still in working order. One friend found an original 1880s Kodak box camera in almost mint condition and still in its original case with all the bits and pieces included and even the original manual, on a shelf in a charity shop in Melbourne, bought it for a very reasonable price and did such a good deal on resale to a Japanese collector that he paid for most of a new Leica M6 with the proceeds. Lucky him, this has never happened to me.
My camera collecting days are now past but I still enjoy visiting charity shops in country towns to see what I can find in boxes or on back shelves. Alas, nothing much turns up. Two recent 'spots' were a Nikkor 300 F-mount from the '60s in okay condition which the shop wanted $120 for - my camera dealer has one in much better nick for $150 so I passed - and a little used Nikon D60 with a $180 price tag but missing its battery and even the viewfinder rubber whatzit which I passed on as not worth spending money on. Interestingly, a few minutes later an old-timer came in and grabbed the D60, happily exclaiming that he would probably get "hundreds" for it on Ebay. One of PT Barnum's clientele, I decided...
I guess the bargains are still out there but it's now increasingly like looking for lost diamonds on Bondi Beach in Sydney. There is always the possibility but a lot of time gets used up just scooping sand...