tychobrahe parapedal
if we're talking about production pedals from any pcb vendor then there's a lot of stuff that deadendfx does that is stupid rare ... lectricfx does a lot of the rare EHX stuff ... and those are still rare even within DIY because they're huge, complex, and expensive projects with rare parts requirements that very few vendors offer boards for
if we're talking sheer rarity, probably original designs ... or any of the vendors' original designs