Berg is pretty new but it's the most unique disc I've thrown. It's incredibly slow -- a true speed 1 disc. It has a headwind-fighting 0 turn, and yet still fades pretty minimally like your average putter mold. And the shape of the disc is really bizarre...kind of a thumb track with big volcano dome in the middle.
The disc shape and flight characteristics are unique, and the speed class is rare. It's a pretty one-of-a-kind mold in my book.
The berg isn't kastaplast's most unique disc though IMHO, the Rask, with its extra wing is much more unique.