Scientist that I am, I gathered data related to the question of scarcest issues in high grade. Here are some census figures, number of copies graded in 9.2 and above:
#1 - 6
#2 - 15
#3 - 2
#4 - 11
#5 - 10
#6 - 6
#7 - 9
#8 - 8
#9 - 14
#10 - 17
#11 - 10
#12 - 10
#13 - 5
#14 - 21
#15 - 11
#16 - 16
#17 - 7
#18 - 10
#19 - 14
#20 - 14
Granted, this data set provides only an estimate, especially since some of the recorded numbers are dual submissions. Still, it is clear that FF 3 is the rarest early issue in high grade, followed by issues 1, 6, 13 and 17. Having been looking for a nice upgrade of 17 for the past 3 years or so, I can certainly confirm its scarcity in the marketplace.