I have a bunch of 1-off's and a prototype that might be considered one of a kind, but they are not rare old PRSi. Many of them have differences with the production models that are only skin deep.
Johnny Hiland 1-off with McCarty switching in Iguana Run,
Left to Right:
HB1 1-off Quilted Purple Burst with Brazilian Rosewood Fretboard and Veneer, Paua birds, and a Piezo
Prototype 305, 1 of 20, but all 20 were different. That one has black MOP inlays, a Modern Eagle Inlay, and tapped singlecoils.
513 1-off Gunmetal Blueburst, The front looks like a charcoal burst, but the burst actually fades to Gunmetal Blue. Brazilian Rosewood Fretboard.
513 1-off Smokeburst. Made at a time when maple necks and fretboards weren't even options on the 513. Has the old 513 neck carve, scalloped nut, and a satin nitro finish. Gold MOP Inlays. I call it "The Romulan," because the birds themselves are nearly invisible against the flame maple. "Cloaked birds," if you will.
And finally, SAS 1-off in Ocean Teal
I also have an special order HB1 in emerald green with a matching back. Back at the time that that one was made, the emerald green HB1's had black backs. I don't know if its one of a kind, but it is uncommon.