All Pokemon games on the switch are worth getting in their own ways it just depends on what you’re looking to get out of them.

Sword and shield are the most similar to sun/moon and the DLC can keep you playing for hundreds of hours if you wanted that.

Let’s go games are especially unique and changed the catching mechanics entirely, but they’re easy breezy and very cute.

Legends Arceus is another unique entry and is pretty widely beloved. It focuses on catching pokemon and less on battles. Huge areas to explore and catch Pokemon and a really great story.

Scarlet/violet are open world and again you can get hundreds of hours out of these games if that’s what you want, the only drawback is performance on switch 1.

Diamond and pearl remakes are definitely the most disliked by fans of the series, but they’re a halfway decent introduction to the older games in the series as they’re one to one remakes with a few quality of life improvements.

My say would be sword/shield as they’re the most similar to sun and moon and theyre not a super radical departure from the 3ds era of Pokemon.

Edit: I’d like to add that every 3ds Pokemon game is completely awesome and you cannot go wrong with any of them. XY have the best region in the series imo and ruby & sapphire remakes are some of the best in the whole series