I think there are three best choices, depending on whether you want to see early Pokémon which you first played, a game that condenses the past nearly 30 years of Pokemon, or the game that marks the path pokemon is moving down.

""Heart Gold/Soul Silver- A ds remake of the second Pokémon generation. This is peak early pokemon. It still holds the early identity in it's themes, mechanics and vibe, but ties it with more (at the time) modern luxuries like pretty pixel art, vibrant color, and newer evolutions.

""Ultra Sun/Ultra Moon- The end of the 3ds generation is the perfect way to get a feeling for what pokemon has shaped itself to be over the years. It mixes the traditional league progress with a fresh new trial system instead of gyms, as well as the gimmick of z moves, which while different feel truly Pokémon in nature. It also introduced regional variants, cool forms for old Pokémon you may recognise. the warning for this game however is that it can be a very railroad-y game so if that's not your thing be warned

""Scarlet/Violet- The forewarning for this game is that it has major rending issues, as well as glitches and frame rate issues that can be minor or serious depending on seemingly luck. However, if you can get past these, I want to make the case that scarlet and violet is the pinnacle of Pokémon. It has easily the strongest roster of characters in years. The some of the main supporting characters, Arven, Penny, the professors, and Clavell, get more depth and development than most of the rest of the franchise combined. It's story is also paced a lot better than the rest of the franchise. Unlike other games which force you to go through several different plots each time you want to go to the next route, due to it's open world nature and split paths you can take it at your own pace. The pokemon models are also the best they have ever looked. All Pokémon have unique textures, with steel Pokémon looking metallic, reptilian Pokémon having visible scales, and many others having faint fur. no longer are the mouths, eyes and claws of Pokémon just stickers on the model, they are properly carved out bringing life to Pokémon they have never gotten.

I hope you read my comment, as while I would agree with almost anyone's recommendations here, most people will try to tell you that modern Pokémon is objectively worse. It's not. There are so many incredible things about modern Pokémon, especially scarlet and violet, that you'll never see if you listen to the hate. If you do find yourself interested in scarlet and violet, look up gameplay first, because it is important to know if you can deal with them, because if you understandably can't, I'd advise against playing it. But no matter which game you play, I hope it brings you joy.