Here's a fun one! I had this conversation with someone IRL earlier but they aren't a Pokémon fan, although familiar with the franchise through playing Gen 3 themselves / watching me play more recent games. They pointed out that they've noticed a recent shift in my preference from Jolteon to Bulbasaur and of course that opened up a can of worms. 
I've been a fan of Pokémon since it first came to NA/EU. Early on my favourites were Pikachu, then Jigglypuff, then Poliwhirl, finally settling on Eevee. It alternated quite fast due to my initial exposure being through the anime. After Eevee my second favourite was a tie between Articuno and Lapras. Ice types and ice type move users (especially Ice Beam) were my favourite. Eevee was my #1 favourite from 1999 all the way through to approx 2010. Then it shared the spotlight with Glaceon.
When Sun and Moon came out in 2016, my favourite became Mimikyu. I even have a tattoo of him on my calf!
Sometime around 2020/2021 my favourite became Jolteon. I can't explain the logic behind it. I just suddenly adored them. Maybe a sudden love of the colour yellow in the later 2010s played a role in this shift.
More recently, my favourite has become Bulbasaur! And it is strange, because I've never even had a preference of the three Kanto starters until now. I might be 25yrs late but I've finally made my choice!!
I still adore all the Pokémon I've mentioned. Just favourites shift over time.
Has your favourite Pokémon changed over time? And if so how so?

I've been a fan of Pokémon since it first came to NA/EU. Early on my favourites were Pikachu, then Jigglypuff, then Poliwhirl, finally settling on Eevee. It alternated quite fast due to my initial exposure being through the anime. After Eevee my second favourite was a tie between Articuno and Lapras. Ice types and ice type move users (especially Ice Beam) were my favourite. Eevee was my #1 favourite from 1999 all the way through to approx 2010. Then it shared the spotlight with Glaceon.
When Sun and Moon came out in 2016, my favourite became Mimikyu. I even have a tattoo of him on my calf!
Sometime around 2020/2021 my favourite became Jolteon. I can't explain the logic behind it. I just suddenly adored them. Maybe a sudden love of the colour yellow in the later 2010s played a role in this shift.
More recently, my favourite has become Bulbasaur! And it is strange, because I've never even had a preference of the three Kanto starters until now. I might be 25yrs late but I've finally made my choice!!
I still adore all the Pokémon I've mentioned. Just favourites shift over time.
Has your favourite Pokémon changed over time? And if so how so?