I think my rarest rifles come down to:
H&R Model 800, the 22LR version of the much more common model 700 22WMR, both of which are Reising designs and derivatives of previous Reising rifles. I rarely see these for auction/sale. 2-3 in the last half decade I've been looking for one.
Then there is the Model 450/451, which several hundred where made in two batches. I have a 1st batch. Only thing special about this gun is the heavy tapered barrel they slapped on a Model 265, I think? It's literally just a regular H&R action and slightly modified (crudely at that) stock.) These pop up from time to time, usually in not great condition.Not sure if the sight blocks are stock or not.
Next is the Model 5200, which is the Civilian version of the M12. Having a Walnut stock instead of a Birch stock of the M12. I don't know if the M12 has the same roll marks, but the 5200 is marked with "22 LR MATCH". Not sure on total production numbers of the 5200, most of these turned out as M12s. So while the M12 is common, the 5200, despite being mechanically the same, isn't. Seen a few pop up in the last couple of years.
And then, also the H&R Model 370 Ultra Medalist, in 22-250. Rarest of the the Ultra Rifle series. Very few of these made, even compared to the rest of the 300 series H&R centerfire bolt actions (using FN, Zastava, and Sakos) The 300, 301, 317, 330, and 340 models are all fairly standard hunting rifles in one form or another with light barrels. The 370 has a benchrest stock, and heavy barrel. What sources I have for the 300 series is 'several thousand' in total made. Including the O'Brien leftovers that are the 317 Wildcat line.
I have not seen another model 370 for sale since I got this one back about 4-5 years ago. It shoots absolutely like the proverbial tack driver.
Less rare items I have include:
317 Ultra Wildcat in 223 rem, but it is not an original O'Biren. Original O'Biren stock model 317's have the knurled dovetail mounts, H&R later production Sako contract actions have normal round top receivers.
301 Ultra in the Mannlicher stock with Zastava action.
330 Ultra FN action.
700 22WMR (and it is smooth to shoot, get one. One of my favorite firearms.)
360 Ultra Automatic Deluxe
760 Saraha (the funny automatic ejection blowback the ATF forced them to quit production of.)
A Buckmark Rifle
BSA Centurion
And a couple of shotguns
H&R SxS Belgian Tooling Hammergun in 28g, case colored
Pre-Model 11 Remington Autoloading Shotgun
Tula TOZ-34 Baikal middle-grade (still trying to find a Sovereign or Souvenir grade)