My favorite 'production' unique handgun is the same as @RickN, the Webley-Fosbery semi auto. I'll probably never see one outside of a museum, but that thing has always been at the top of my weird/unique handguns I'd like to have.
If were talking any handgun, even if they are one of a kind guns, I'd have to say it'd be the Cabot Meteorite pair of 1911's. A pair or LH and RH 1911's known as the 'Big Bang Set'. Bimetal support only to the frame/slide rails, hammer, guide rod and extractor. A few steel parts, such as the barrel, springs, and pins. All of the other components are crafted from a single meteorite. $4.5 million takes them home.