What makes a gun unusual? Age? Engineering? Baubles and bolt-ons?
I'll go with age and engineering, a 1920s Ortgies 7.65mm. Striker fired, 27 total parts, no screws or bolts.
And then there's baubles and bolt-ons. Ruger MKIII 22/45 with Millett red dot and Hinterstown suppressor, along with some internal changes.
But then there's cool just because. Walther TPH 22 German import post-GCA68 with custom grips.