Don't know about fan favourite but Decidueye is my personal favourite Alolan starter, as is Feraligatr my favourite Johtonian and Swampert and Blastoise are my tied favourite's from thier home regions too.
Decidueye is considered the fan fave in non-Japanese territories, yet Japan favours the other two Alola starters. Galar seems to have a similar regional opinion difference, with Cinderace the fave of the West while Inteleon is the fave in Japan.
After clear analysis, I feel these are considered the so-called “fan faves” of their region.
Kanto: Charizard (duh)
Johto: Typhlosion
Hoenn: Sceptile (anime likely influenced this one, as I think Blaziken was the fave before)
Sinnoh: Infernape (at least for final stages, given Piplup outshines Empoleon)
Unova: Samurott (I’d say Serperior was the original fave, but LA really boosted the Oshawott line’s popularity)
Kalos: Greninja (duh)
Alola: Decidueye (at least in US, Japan prefers the other two)
Galar: Cinderace (again likely due to anime influence, Japan instead favours Inteleon)
Hisui: Hisuian Typhlosion (I believe it’s because it receives way more fanart than the other two)
Given the Fire-type is often associated with cool designs and being the strongest of the starter types, you can see why Fire-types dominate the general fan perception.