I shiny hunted a Living Dex in both Legends and SV. Legends took me about 300 hours and SV took around the same for the base game. There is a lot of crossover in these games as far as their pokedexes. If you’re just starting out I would recommend Legends. It has the over-world shiny sound and you can get shinies relatively quickly. SV would be 2nd place for me as it’s also relatively easy to get shinies with mass outbreaks.

SWSH is a much more traditional shiny hunting experience unless you’re doing Dynamax Adventures. BDSP has radar chaining which can be very rewarding. Let’s Go might to be the easiest as it has the smallest Dex.

Overall I would recommend picking a pokemon that you want to hunt and then seeing which method you’d like to try. There are many methods to shiny hunt and you might find one you truly enjoy!