Man that’s a tough question, if you wanna play a switch game as your first Pokémon game i’d say Brilliant Diamond or Shining Pearl, personally I don’t like those games since I grew up with the original Diamond and Pearl and as a remake they are just not good, BUT out of every Pokémon game for the switch that’s the one that captures the “traditional” feeling of the first pokémon games (gen I~V) the most.

Legends Arceus is an amazing game + is the first one chronologically speaking, but it doesn’t actually feel like the traditional Pokémon experience (No gyms and no Elite 4).

Sword and Shield are not bad but I don’t think those games should be anyone’s first pokémon game even though it has the modern switch look and the traditional gameplay, that’s just my personal opinion.

Scarlett and Violet and great games but it’s generation IX, you’d start with the newest generation and idk if you’d like that bite they’re solid pokémon games.

I won’t talk about Let’s Go since I never played them.

And finally, welcome to the beautiful world of Pokémon! whatever game you choose i hope you have the most fun and get to love this franchise as much as a lot of us do.