What is the rarest set in Fortnite?

Excuse me, could you please clarify something for me? I've been hearing a lot about Fortnite lately, and I'm curious about the rarest set in the game. I'm sure there are many different sets available, but which one is considered the rarest? Is it based on the limited availability, the difficulty to acquire, or perhaps some other criteria? I'd really appreciate it if you could enlighten me on this matter.