How about a Beam Spring keyboard that works on modern PCs?
Hey, I've got one of those, hacked together the adapter myself. It's wonderful to type on, but about as far from being portable as you can get. There's at least one other member working on a similar project, but via replacing the controller. Perhaps the only one or two available in the world at the moment.
I'd like to get a uTron myself, though based on reports of the keys not being full-size, probably wouldn't actually use it. More interested though in getting one to try it out. I've got a DataHand already, otherwise it would probably be near the top of my list.
I'm with Ripster on this one. The industrial IBM Space Saving Keyboard probably wins top place. I was outbid on the only one I've seen for sale (the one sold by Webwit on Deskthority). I think second place would probably be Ripster's Financial Keycap SSK.
Maybe the rarest though, from what I've seen, would be an IBM 2260 terminal keyboard. I haven't yet seen a single one for sale. I guess I'd actually prefer to have one of those over an industrial SSK, but I doubt many others would share my enthusiasm, thus with respect to geekhack members in general, I doubt this would be anywhere near the top of the list.
http://www.columbia.edu/cu/computinghistory/2260.html