Personally, mine is joint first between Sandshrew (the cutest), Espurr (most relatable), and Gastrodon.
After those, in no particular order, I like Turtwig (best starter, fight me), the Roggenrola line (little rock guys), the Litwick line (candles and lamps with nefarious intentions), and Metagross (cool robot thing).
Kyogre is the best legendary in my opinion
Nidorina.
It’s one of the first Pokémon in gen 1 that isn’t recognisable as another creature. It feels like the first ‘monster’ you really run into.
I also caught one early in my first playthrough, and didn’t find out about the moon stone evo until very late game. I just thought that Nidorina was the final form. By the time I learnt, I was kind of attached to the middle form.
The Nidoran families are really cool! I personally really like Nidoking, since I used it in both my gen 1 playthroughs (leaf green and Let’s Go Pikachu).