If you haven't played since the Gameboy days, honestly...Brilliant Diamond or Shining Pearl might be good games to get back into the swing of things. They'd play the most like the games that you remember, so you can re-learn the type weaknesses and gameplay tropes before trying out any of the newer games. They get a lot of flak and bad reviews, but they're not really bad games. They're just disappointing remakes since they didn't include the improvements from Platinum for the DS. They make a few weird choices, and there IS a difficulty spike once you reach the Elite Four, so you're going to have to grind for a while to actually beat the game. But, overall, it might be a way to ease you back into things.
Sword and Shield are kind of a link between the "old" and "new" styles of Pokemon. It's fairly linear, but there are a few big open areas. It's story isn't anything to write home about, but it's got a few impressive looking areas, here and there. Some Pokemon are visible in the overworld, and some are still randomly found in patches of grass. If you want something a little more modern, but still firmly rooted in the gameplay you know, this wouldn't be a bad choice. If you're willing to try something a little bit different, I'd give this one a try.
Scarlet and Violet would be the next step towards the "modern" style of Pokemon. It's open world, but there's no level scaling, so you are encouraged to take things in a certain order. If you take the intended order, you basically end up swinging back and forth between the left and right side of the world map, which means a fair bit of backtracking in the early game. You may actually get lost, that's how big the world map is. And ALL the Pokemon are now visible on the overworld, without any random battles. But, it's probably got the best story out of any of the Switch Pokemon games. And the characters are great! Some of these characters have become some of my favorites in the entire franchise. There's also the bugs, which have mostly been ironed out by now, but it still doesn't perform the best. Be sure to save often, just to be sure you don't lose any progress, and be sure to download ALL the patches before you start playing. This one might challenge you in some ways, but I do think it's the second best Pokemon game on Switch.
Legends Arceus is my personal pick for the best Pokemon game on Switch. But, it's also dramatically different than anything else the franchise has ever attempted to do before. The focus isn't on battling in this game. The focus for the majority of the game is on catching Pokemon. You have to catch the same Pokemon multiple times in order to level up the Pokedex and gain ranks and abilities, which is more fun than it sounds. You can catch Pokemon without ever actually going into a battle. You can craft your own items. You get to watch your "hometown" expand and grow. It's a breath of fresh air. But, it's also VERY different from any other game in the series. I think it's worth trying out, but maybe only after you play one of the other games, at least. This game is....almost more for players who have grown bored of Pokemon's formula? It mixes things up so much, that you might not even consider it to play like other Pokemon games. Even the battle system is slightly different. So, I probably wouldn't recommend this game as your first one, but....maybe try it out eventually.
Personally, I would probably avoid Lets Go Pikachu or Eevee, for now. For most players, the control scheme is kind of annoying. The motion-controlled capturing mechanics are kind of forced on you, no matter what. It's not a bad game, the controls just take getting used to. And otherwise...it's Pokemon Yellow. I think all the other games on this list are more interesting, personally.
So, to sum up, I'd say go for Brilliant Diamond or Shining Pearl if you just want to re familiarize yourself with the basic Pokemon gameplay mechanics. Or Sword and Shield if you're up for something slightly more modern, but still accessable. Then once you've found your footing, maybe try out Scarlet or Violet. Or if you're up for more of a challenge and a change of pace, maybe try Legends Arceus.
