OshyHikari
c l a r i t y
From what I was able to gather, the most popular starter to least popular for each generation was the following, based on appearances in marketing and fandom opinions, though some generations did not have an outright standout as I found out below:
Gen 1: Charizard > Blastoise > Venusaur, obvious due to Charizard having two Megas, and getting its Gigantamax form before the other two
Gen 2: Typhlosion > Feraligatr > Meganium, due to Cyndaquil line being owned by two main characters in the anime (Ash and Dawn), as well as the main protagonist in the Johto manga chapters
Gen 3: Blaziken > Sceptile > Swampert, obvious because Blaziken got its Mega before the other two, and I recall Treecko being chosen more than Mudkip based on design preference
Gen 4: Infernape = Empoleon > Torterra, given Infernape's major role in the anime and Piplup's heavy appearances in marketing
Gen 5: Samurott > Serperior > Emboar, based on how hated the Tepig line is by the fandom and how much Oshawott featured in marketing.
Gen 6: Greninja > Delphox > Chesnaught, obvious enough for reasons already mentioned. Chespin line is hurt severely by the overwhelming hatred towards Quilladin
Gen 7: Decidueye = Incineroar > Primarina, namely due to Primarina being seen as too "feminine" while the other two got major appearances in Pokken Tournament and Super Smash Bros. respectively
Gen 8: Cinderace > Inteleon > Rillaboom, the former was the first one obtained by Goh in the anime and is also favoured more in competitive, while Inteleon is more divisive outside of Japan, and Rillaboom isn't liked much outside of competitive circles
As you can see, the Fire-type tends to be most favoured by the marketing team when it comes to a generation's starter trio. The only time Grass was the most popular was Gen 7, as I recall Rowlet winning a poll for most chosen Gen 7 starter, but Incineroar gets fairly equal treatment too so I put them as even.
Gen 1: Charizard > Blastoise > Venusaur, obvious due to Charizard having two Megas, and getting its Gigantamax form before the other two
Gen 2: Typhlosion > Feraligatr > Meganium, due to Cyndaquil line being owned by two main characters in the anime (Ash and Dawn), as well as the main protagonist in the Johto manga chapters
Gen 3: Blaziken > Sceptile > Swampert, obvious because Blaziken got its Mega before the other two, and I recall Treecko being chosen more than Mudkip based on design preference
Gen 4: Infernape = Empoleon > Torterra, given Infernape's major role in the anime and Piplup's heavy appearances in marketing
Gen 5: Samurott > Serperior > Emboar, based on how hated the Tepig line is by the fandom and how much Oshawott featured in marketing.
Gen 6: Greninja > Delphox > Chesnaught, obvious enough for reasons already mentioned. Chespin line is hurt severely by the overwhelming hatred towards Quilladin
Gen 7: Decidueye = Incineroar > Primarina, namely due to Primarina being seen as too "feminine" while the other two got major appearances in Pokken Tournament and Super Smash Bros. respectively
Gen 8: Cinderace > Inteleon > Rillaboom, the former was the first one obtained by Goh in the anime and is also favoured more in competitive, while Inteleon is more divisive outside of Japan, and Rillaboom isn't liked much outside of competitive circles
As you can see, the Fire-type tends to be most favoured by the marketing team when it comes to a generation's starter trio. The only time Grass was the most popular was Gen 7, as I recall Rowlet winning a poll for most chosen Gen 7 starter, but Incineroar gets fairly equal treatment too so I put them as even.