One of a Kind is a very dark green creme that looks black in quite a lot of lighting conditions. Despite how dark it is, One of a Kind took three coats for full coverage because it's a bit jelly-ish. It's also very shiny, but since I almost always use top coat that's less of a selling point than it might be.
I've only started collecting greens in the last year, but even so I was pretty sure I had something like this color in my stash, and lo and behold, look what I found:
In a lot of lighting conditions, I'd be hard pressed to tell these apart. The Milani is a teensy bit more yellow than the OPI and a teensy bit darker than the Zoya and a bit more jelly-ish than either of the other two, but unless one is a really serious green lover, I don't see that having all three of these is essential. Polish or Perish reported that NYC Flat Iron Green is in the same range as well.