Going off of the floor at indoor ranges, I'd say out of all the brass I normally find (.380, 9mm, 40, 45, 10mm, 38, 357)
The least common that I find is .380 and 10mm. So out of modern, "common" ammo, I say those two. Personally I don't care if .380 dies, because I don't like it myself. I would like to see more 10mm around though.
so, coming from a different angle, thats my take on that.