Let's kick around some rarities from the modern era, besides the 'rainbow' of ultra-modern limited edition cards.
1991 Kayo Signature Series
Kayo boxes and packs advertise autographed cards. TCDB has images of the following:
014 Charles Williams
015 Meldrick Taylor
089 Donovan Ruddick
180 James Toney
It also lists 190 Carlos Palomino but the photo is of a regular card. Jeff Allender's checklist site also lists the Palomino but without a picture. Not sure if the card actually exists. Please post a pic if you have one. I have developed my doubts given that the others were all active at the time.
There is also an unsigned Pernell Whitaker known.
The year before I published my last guide, signed cards of Prince Charles Williams and James Toney both surfaced (at the time, 3 copies of Williams, 2 of Toney). I scooped up 2 of the Williams cards and one of the Toney cards at the time. I've since sold one of the Williams cards but still have a Williams and a Toney. I am not sure whether the Williams that sold on eBay last week is part of the recycled find.

2016 The Greatest Boxing Ringwalk Songs
An interesting UK set icreated as a promotion of a CD compilation of music. The company selling the CDs, Split Records, advertised a free set of cards with online purchases, while supplies last. My guesstimate is that the print run was well south of 200 sets. As far as I can tell, PSA has never graded one. These were gobbled up fast and never seem to surface. Fantastic checklist of fighters at the time and even one of Brad Pitt as Mickey, the Gypsy champion bareknuckles boxer, in the Guy Richie film
Snatch. Speaking of Gypsy fighters, the set also has the first card of Tyson Fury.