What is the hardest cosmetic to get in Sea of Thieves?

So, for those avid gamers of Sea of Thieves, I'm curious to know, what stands as the most elusive and difficult cosmetic to acquire within the game's vast offerings? Is it a rare item from a limited-time event that's now all but a memory? Or perhaps a hidden treasure only obtainable through a series of intricate puzzles and challenges? The quest for this elusive cosmetic must be quite the adventure, and I'm eager to hear from those who've braved the seas in search of it.