OK, you've got me hooked.
The rarest
production baritone sax that I'm aware of would be the curved horn from
Pelisson (the horn might also be pitched in F, which also makes it very interesting).
There are a few one-off
straight baritones, which would be considered rare.
Prototypes are rare, too. I've heard that there is a King Super 21 Silver-Sonic bari out there, somewhere, but I haven't been able to confirm one.
There are other production horns that were produced in a specific metal, plating or finish that would be considered quite rare, like Buescher's ONE set of sterling-silver True Tone or Aristocrat horns (don't remember off the top of my head which model).
A. Sax baris are rare, because of how many were produced and how few still exist, but I've seen and heard quite a few.
There are a few makes/models that are rare because of how few there were, like, say SML or a Buffet S1 Prestige in copper.
So, I'm intrigued ....