The most sought after set in US philately is the 1930 Graf Zeppelin set of 1930 (Scott # C13-C15). For the last eighty years it has been the alpha and omega of stamps- the set every young collector always wanted. Ironically, acquisition of the set has been one of the great killers of collectors. So many people get back into the hobby and effectively end their stamp buying when they finally purchase a Graf Zeppelin set because they have now gotten the set of their dreams.
Nearly 250,000 sets of the Graf Zeppelin set were issued. Quite a few were used on specially prepared Zeppelin covers, the reason for which the set was released. Probably about 150,000 mint sets exist. This is the same number of sets as were issued to collectors of the 1999 Recalled Legends of the West sheet (Scott #2870) that were issued to collectors after it was discovered that the sheet had been designed and printed with the wrong name assigned to one of the portraits, and the vast majority of the set was destroyed.
So there are approximately the same numbers of mint sets of Zeppelins as of the Recalled Legends sheets. Each is needed by every US collector who aspires to complete a United States stamp collection. A nice Zeppelin set sells for $1000. A nice Legends sheet sells for just a bit more than $100. Obviously story, history, and cache matter a great deal, and the Zeppelin set has that. But the Legends sheet isn’t chopped liver either; it has a pretty good story too. I can only speculate that over the next twenty years that the Legends sheet and the Zeppelin set will narrow the range that separates their price, and I have to believe it will be more with the Zeppelin set coming down than with the Legends sheet going up.