I know some people look for specific cats that look like their irls or fancats, etc. but I mean the cats that are specifically sought after on PCE as "rares" like albinos, voids, minks, the like.
I think voids are definitely the hardest because you've got five if not six genes to select for: the solid pattern, the no whitespotting, the maximum 4 color density, the full color strength, the black color group, and sometimes the wind because black watercolors typically aren't considered voids.
I guess the one other very very hard to breed cat would be the goldbino, because you need an albino to start with and an inverse white cat and both of those are rare/expensive/both.
What do you think?