The scarcest US commemorative in mint condition is the 10c orange perf 10 #404. Fewer of the higher value dollar denomination Columbian and Trans-Mississippi were issued than #404 but far more of the 10c were used on commercial mail and thus used and lost to collectors. Here's the story: For many years leading up to the issuance of #404 the Post Office had been receiving complaints about the perforations on United States postage stamps. The perforation gauge that was used was perf 12 and this was just too close resulting in many sheets of stamps separating on their own often even in the postal clerk's drawer. The simplest solution for this was to change the guage of the perforation so that there were fewer perforations per row and thus stamps would be more difficult to separate. This was tried for the first time on the Pamana-Pacific issue with the stamps being first issued in perf 12 (#397-400A) and then, as an experiment, in perf 10 (#401-404). No great fanfare was made over this experiment and many collectors ignored putting a mint copy of this stamp in their collections. This resulted in real scarcity of mint examples when collectors discovered the omission. By the way, perf 10 wasn't very successful either. It made stamps too difficult to separate (as the many pulled perfs on perf 10 issues attest) and the Post Office eventually settled on a gauge in the perf 11 range.
