When I was much younger, I only knew about the Kanto region for the most part. I watched the Indigo League anime and I owned a copy of Fire Red (which I know is technically Generation 3, but it primarily had Gen 1 Pokémon for the most part.)
Back then, my favorite was initially Magmar, simply because it looked cool and I liked fiery things, but when I first encountered Mewtwo in Fire Red, it instantly became my favorite for the longest time.
That is, until I got Pokémon Diamond, it was then I started learning new Pokémon, and for a brief time, Dialga was my favorite, but I only knew about the two games back then, I didn't have the internet.
Around when Gen 5 came out, I got some huge book of all Pokémon across the 5 generations for Christmas, and I've discovered a lot.
It was also there I found out about Deoxys, and I was extremely intrigued with the idea of a Pokémon that's just a straight-up alien.
Whenever I got access to the internet, I'd do research on this Pokémon, and it quickly became my all-time favorite, almost as if I were connected to it in some ways.
Especially in the Destiny Deoxys movie. That's definitely my favorite of the Pokémon movies.