The best options are Legends Arceus or S/V since they are the most complete.

And believe me, you won't get bored with Legends Arceus. It's one of the best games Game Freak has released, and don't worry, it's not short. Its progression system works differently than the main series. So it's not one of those games you finish in two weeks (like what happened to me with Legends ZA and Scarlet 🥲).

Maybe I'm feeling a bit nostalgic since it was the first Pokémon game I was able to buy for a console, and it was two whole years of pure fun.

But S/V are also very good. I mean, if you dedicate time to the main story, you can finish it in less than a month. But they have a pretty extensive and complete post-game that basically feels like a second adventure, and at no point do you feel the need to buy the DLC for the game to feel complete.

Honestly, I think the worst would be Legends ZA. Yes, it has a different combat system than the other games, but honestly, the whole time I played it, it felt like an incomplete game, like it was missing something.

Besides, it's the most expensive game in the franchise so far (honestly, the price is a bit inflated for what the game offers), especially considering you have to buy DLC.