What is the rarest shiny in S/V?

Question

I need to know which Pokemon I should prepare for when filling out my shiny living dex.

Comments

Three Segment Dudunsparce would be up there.

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Most sources claim shiny Sinistea is one of the rarest shinies in S/V.

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Maushold Family of Three is also rare. Roaring Moon is pretty difficult too I think because of cluster spawns in the cave where you find it.

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Three segment Dudunsparce, Maushold family of three, and antique sinistea all have a 1% chance of occurring. Dudunsparce and Maushold may be rarer because you have to evolve them.

Answer 1

There are 3 options for Rarest Shiny Pokemon (that isn't shiny locked):

-> Maushold family of 3
-> 3 segment Dudunsparce
-> authentic Sinistea/Polteageist

The first 2 are both forms that have a 1% chance of occurring at the time of evolution. Whether they evolve into their more common form or the rarer form is completely out of the hands of the player -- it's based on hidden information that the player can't even see, and nothing the player does can change it.

Sinistea/Ploteageist are a little different situation. The Pokemon is inhabiting an antique tea set, the originals of which are extremely rare and therefor valuable. But a multitude of knock-offs were created in an attempt by others to cash in. As a result, most Sinistea/Polteageist are the Phony form. The significantly rarer Authentic form can be determined by locating a tiny stamp of authenticity on the rim of the underside of the teacup/teapot. Unfortunately, there is no way to manipulate the camera in such a way as to locate the mark, when encountering them in battle or in the world. Your only option is to simply catch every one you come across, then check it's summary and rotate the camera to examine the rim on the underside. Even worse, you can't breed Authentic Form down -- any offspring of an Authentic parent will automatically be Phony Form.

Comments

You can see Authentic stamps in the overworld, you don't need to catch every Sinistea.

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I don't see a source for this answer.

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The answerer took the information from the comments and attempted to convert it to an answer.

Questions like this deserve extensive research, especially in a game like Scarlet and Violet, where the encounter tables are far different than any past Pokemon game. There are also the possibilities of rare spawn Tera Raids with certain typings, which also have a good chance of hosting the rarest shiny, especially because Shiny Charm and encounter powers do not affect them.

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@This Guy -- people source experience with nothing further to back it up all the time.  It really doesn't add anything to an answer to type that in.

@McRibFarwellTour -- I did not convert the comments into an answer.  Though I did read them before putting my answer together, I already knew that was the answer.  And stating that Tera Raids can have anything in them doesn't change the answer.  Scripted Raids are a completely separate topic.  And the typing of the Pokemon is also irrelevant to the question.

I am curious though, how can you see the Authentic stamp in the overworld?

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This isn't the kind of question you can answer with experience, unless you are a dataminer or something. Questions you can answer with experience are like "Where is x in Pokemon Sword?" or "How can I do x in Pokemon Ultra Moon?"

I'm talking about specific Tera Raid Pokemon that have different Tera types that could be shiny, or have a certain egg move. For example, a Level 30 Shiny Sliggoo with a specific Tera Type. Goomy doesn't evolve into Sliggoo until Level 40, so this would be extremely rare.

If you turn the camera around to the back of sinistea, and then down so you can see "into" the cup from the bottom, you can see the stamp if it's there. Sinistea won't run into you, so it's harmless to take your time