A lot depends on what you mean by "a lens which does something others can't", and on whether you want/need that application. In other words, how unusual is the image, either for 'character' or for sharpness, and what combination of physical advantages does the lens have (small size, big aperture, fast handling...)?
The most unusual lens I use on my Ms is a Thambar, which is unique in being a rangefinder-coupled soft-focus lens. But if you don't like soft focus, it's a waste of quite a lot of money. And I use an old 35 Summilux (bought new in 1984 or so) instead of a 35 Summicron because while the Summicron is better at f/2 to f/4, by f/5.6 it's hard to tell them apart and at f/1.4 there is no contest. I had both; I got rid of the one I saw no reason to keep.
Cheers,
R.