We might think that Fortnite's rarest skin is the Ghoul Trooper — released at the beginning of the game. However, do you know the Recon Expert? Appeared more than two years ago, on November 12, 2017, this skin is undoubtedly one of the rarest in the game. In addition to the fact that it's no longer available in the Item Shop, the Recon Expert was a rare skin (blue),so nothing exceptional on the surface.
As a result, very few players had taken out their wallets to buy them. Except that since then, the game has exploded and the skin has become the Holy Grail, exclusive to the oldest players. In general, we notice that the rarest cosmetics aren't necessarily the legendary ones, but rather the most common or the rare ones. What creates the rarity of an item is the number of players who own it.
Unfortunately, there's no point in hoping one day to see the skin again in the Item Shop. Unless there's a policy change for Fortnite, this skin should remain forever THE skin to impress your in-game opponents!