On SO, only 172 people have Legendary. Only Reversal, with 184, is close to it in rarity. Only 6 people have Legendary here on Meta. On Travel, it has never been awarded.
To spare me going through all 100+ sites to verify my hunch, can someone demonstrate either that it is the rarest badge (possibly tied at 0 with other rare badges) on every site in the network, or provide me a counter example?