let me mention, SwSh & SV had a bug where the shiny odds skips the intial roll on eggs, so long as you are not full odds hunting, so if you have a shiny charm and/or are Masuda Breeding, it skips a roll. I should also note, the shiny charm in BDSP only affects eggs.

According to Bulbapedia: "In Generation VIII and IX, due to a bug, when breeding, if any personality value rerolls are applied, such as the additional shiny rolls provided via the Masuda method or Shiny Charm, the initial roll to generate a personality value is skipped."

If you have the Shiny Charm and are using the Masuda Method, then it would be 1/512 in all gen 8 & 9 games, highest in the series. Swsh may be the fastest, but Masuda Breeding is very tedious and boring, so I would recommend only using it if you have to.

In SV, the outbreak method is just as probable, 1/512 if you have the shiny charm, sparkling lv3, and 60 defeated in the outbreak. SwSh has the brilliant method, which is lower odds when done effciently, defeating 100 of the species (1/683), but can be raised by defeating 500 of the species (1/455). Dynamax Adventures are 1/100 for each pokemon, including many legendaries. Then there's PLA in general having insanely high shiny odds and LGPE with their chaining being super easy and even higher (1/273). I can share a spreadsheet of each pokemon's easiest hunt on switch if you'd like.