Rarest? Gotta be The Monster, my ADC 8" -barrel double derringer chambered in 7.62 x 39. I had to custom-order it, ADC had to get a reamer to chamber it, and they informed me that NO-ONE else had ever ordered a gun like it.
It's the only one on the planet, and it's MINE. (Like anyone else would want the silly thing...
)
Rarest/coolest is probably the Shansei Arsenal .45 ACP Broomhandle Mauser. Mine's #19xx of about 8500 total produced in the early 30's. Obscure by virtue of caliber, as everyone's heard of Broomhandles.
Gotta pair of Colt Lightning rifles from 1884 and '91, these are pretty rare, and mighty obscure.
A pretty obscure 9mm is the blowback-action Astra 600. Cool appearance, too, although most folks would call it 'fugly'.
Not particularly rare, but fairly obscure and over-the-top in coolness is my pair of Remington M-81 autoloading rifles. They just look really cool, very "artillerish", to coin a word. Cool design, too, a long-recoil action from John Browning's fertile brain.
Obscure to the point of ridiculous: a 1912 Steyr-Hahn auto pistol from the Austro-Hungarian Empire. Loads with stripper clips, no less.
Plenty rare and very cool is the Automag V, a factory-ported stainless longslide auto pistol from AMT. The lightest of the .50 A.E.-pistols, I have seen 2, ever, in 8-9 years of doing this gun collecting thing. I bought the first one.