We know as a whole trio, Kanto is the most popular, but this is explaining which starter is the most popular for its respective region/generation.
Incineroar I can definitely disagree with for Alola, as Decidueye gets far more better reception than the other two. Heck, even Rowlet is more popular than Decidueye itself, which is the likely contributing factor to why Ash’s Rowlet was given the Piplup treatment and never evolved. However, Japanese fans tend to favour Incineroar and Primarina rather than Decidueye.
Unova I feel is not as obvious lately. Serperior was the initial favourite of the trio, however I feel Samurott has caught up fast to it lately. Legends: Arceus I feel massively contributed to Samurott’s recent boost in popularity.
I don’t think Rillaboom is the favourite for Galar. I recall Cinderace being far more popular, especially among the furry fandom, fan artists and being Goh’s partner Pokémon in the anime. If a certain starter is prominent in the anime, expect that starter to outshine the other two. Inteleon however is more popular in Japan than in the US, where it’s more divisive.
Typhlosion being the possible favourite for Hisui I can see where you are coming from. When I see fanart of the Hisuian starters on Twitter, Typhlosion got twice as much fanart as the other two, particularly from Japanese fanart users.
Charizard, Sceptile, Infernape and Greninja can all definitely be agreed with.