I think my personal favorite would have to be the September MC of 2007. In particular the Superior form. I think it's unique because we have so many new items that are the same thing but just a different color, like the pale marionette vs. the crimson marionette, or checkmate vs. checkmate void, but superior form has yet to be duplicated. It is a cosmetic 'skin' item, rather than a cosmetic wearable item, much like uncanny form, but they don't share any similar skin styles, making it like a one of a kind item.
I guess the other reason I like it is just because it has multiple skin colors in it's different forms, so you can use it to it's best potential when making an outfit.