Rarities? I have one uncommon, possibly rare, camera:
Chevet-Wild Endoscopographe. s/n 006. An incomplete endoscopic camera I bought for its lens, a 90/6.8 Boyer Beryl. 90/6.8 Beryls don't turn up often.
Also a few apparently very scarce lenses. To name a few, in focal length order,
45/9 Carl Zeiss Jena Goerz Dagor. In a barrel with a six-bladed diaphragm. Tiny lens, not catalogued. Arne Croell tells me that according to Thiele mine is one of a batch of 25 lenses made in 1937 to a design dated April 9, 1934. Thiele lists one other 45/9 CZJ Goerz Dagor made in 1930 to a design whose date is unknown. Covers 2x3.
S.F.O.M. type f = 100 - n = 5.6 - s = 127 x 127 mm s/n 59 An uncommon aerial camera lens. I have no idea how many were made but it has a low serial number.
Ilex 180/2.8 Acuton s/n 209 Uncommon long normal lens for 4x5 or short normal lens for 5x7. Again, I have no idea how many were made and, again, it has a low serial number.
If you use few occurrences on eBay as an indicator of rarity, well then, 1.75"/2.8 Elcan, 60/14 Perigraphe, 75/14 Perigraphe, 4"/2.0 Taylor Hobson Anastigmat, 1052.8 Era-7, 6"/1.9 Dallmeyer Super Six, 180/6.8 Fotokopist Spezial Reproduktions Optik, 200/2.0 S.F.O.M, 12"/4.0 Taylor Hobson Telephoto, 900/10 Boyer Apo-Saphir. I've sold the really valuable ones.
For the leicanuts among us, I have a few cameras made by Graflex Inc. and predecessors that are probably rarer, in the sense that fewer were made annually, than contemporary Leicas. My 2x3 Graflex RB Ser. B., is one of 9.460 made between 1923 and 1948. Sorting out Graflex press cameras by type is hard because starting late in 1949 Graflex lumped Crown and Speed Graphics in their serial number lists. My two 2x3 Pacemaker Crown Graphics and one 2x3 Pacemaker Speed Graphic are three of around 36,000 made between 1947 and 1955. Certainly more Crowns than Speeds. My little Century Graphic is one of around 35,000 made between 1949 and sometime in the late 1960s.
Um, Sparky, #3A Folding Pocket Kodaks are abundant even now. Go look on eBay. Even Specials, with the expensive ones with various versions of f/6.3 6.75"-or-so Tessars.