has to be shinies, most of all square shinies because holy crap they're rare lol
Pokérus since they are quite rare like shinys I believe and can help by constantly helping you farm iv’s or ev’s can’t remember which one tho
Shiny because you have a lower chance of getting one compared to a legendary which is guaranteed in a game
A legendary, because they're powerful, rare, and valueable
In sword and shield out of those options I'm going to have to say being shiny honestly. Since pokerus just makes them level faster and that is zero issue with candies, there are only a few legendaries you can get without pokemon home. And shiny hunting is still a big commitment, especially with ultra shinies being a thing now.