There are hundreds upon hundreds of branded names out there. There is no taxonomy for these kind of names, so literally the same zoa can have 4 branded names based on seller.
Rare and expensive are not mutually inclusive in that just because something is expensive it is therefore rare. Often times new stuff is expensive just because its new and there's a lot of excitement (hype) around it.
The rare stuff is often hard to keep, doesn't frag or ship well or is prone to suddenly melting. If you're talking truly rare then you'd be looking at stuff like:
Stratospheres
Flaming Mohicans
Wolverine Palys
JF As-kraks (these are becoming more common but are extremely temperamental).
GB Fantasy zoas
ARC Poinsettias
Day-glow Volcanos