Which of the Switch games are the best? I already have BD and it's good but not as good as I remember the DS ones being. Are the Eevee and Pikachu ones any good? Haven't played Sword/Shield and Scarlet/Violet.
Sword & Shield feel like a distilled, refined version of all of the preceding DS & 3DS Pokémon games, with the deep parts getting deeper, and the streamlined parts getting even more streamlined. People won't like every design choice (like I think the routes are too short), but it really feels like the culmination of everything that Game Freak tried to do for decades before on worse hardware, except that this time they were finally able to execute their vision. Again, not everybody likes this vision (and they can't wait to let you know it), but if you like Pokémon games in general, I think that you'll like these games.
Let's Go Pikachu & Let's Go Eevee are meant for a different audience: little kids. Their presentation (art, music, etc.) is fantastic, but the gameplay loop is really cut down and made more casual. Wild encounters are no longer battles. You just try to catch the Pokémon that you find by throwing balls at them, like in Pokémon GO. Some people like that change because of the gameplay variety that it offers, and they like the more-relaxed feel of the game overall. But it this kind of game isn't what you're looking for, then it really won't feel like it. Also, these games go through Kanto again, which lots of people are tired of.
Scarlet & Violet are open-world games. The change in direction definitely feels different from all past Pokémon games, even though they're still quite similar. There are still about a dozen main towns, most of which have a gym leader that you should fight eventually, and you still run around getting into battles and building your team in hopes of beating the champion eventually, and maybe stopping some rivals or a villainous team along the way. The biggest change is in being able to set your own direction for where to go and which main stories to pursue. The games have graphical shortcomings and glitches, the classic aesthetics of how things are presented to you (like in menus) feel stripped down, and parts of the open world feel empty. But if you want to try exploring on your own, no past Pokémon game does it like these ones do.
I'll leave the descriptions of Legends: Arceus to the people who have more experience with it than I do.