Pokémon

If you don't have a Switch OR 3DS and are only going to choose based on the Pokemon games, it's a bit tricky because there's tons of good options.

Switch is a current and active console getting ports and games and content so if you get it, you'll be able to keep up with any new Pokemon games and see where you want to go from there.

3DS has support for any pre-Switch game thanks to emulation, backwards compatability, and the 3DS library and you can do whatever you like there.

As for strictly games, it's really a toss up between what you like. The 3DS era and Switch era both have a lot going for them as well as flaws.

XY are a bit mediocre because they were the jump to 3D and had to make some compromises to even get the game out, so while they were hugely ambitious, they are also very lacking in content and never got a third version but still fairly solid games. I wouldn't say they are the best for getting into newer titles though since they still are a decade old and flawed and there are better places to begin.

ORAS are hailed as some of the best remakes since aside from a few things missing from the original games, they add so much extra and new content and fun that they're pretty beloved. If you like Emerald and want to now just do the same region in 3D, it's not a bad choice.

Sun/Moon/Ultra Sun/Ultra Moon are... odd. I personally love them, but they were a little controversial at release because they are more linear than other games in the series, but they do a lot of really great things and popular additions like regional variants of Pokemon. It's the ultimate 3DS Pokemon in that it has all Mega Evolutions, Z Moves, Gen 1-7 Pokemon, and a lot of fun content to play. The real downside is just that it's linear, and Ultra versus regular is confusing. The Ultra versions add more content and are overall better but change the plot of the game for the worse in kind of ruining the story at the end for the sake of justifying being a revision. I'd keep that in mind when making a choice, though you could just play Ultra and watch the original game cutscenes later. They're overall though very solid!

SWSH is probably the most controversial game here and the first of the Switch entries. It isn't an awful game and it is very easy to get into as a beginner but it's more linear than SUMO and USUM, it introduced "Dexit" (no longer having every Pokemon ingame), and it is pretty mediocre in many areas. It's more disappointing than horrible, so if you're new you probably will enjoy it, but it essentially feels like a 3DS era game lacking features of the 3DS but playable on the Switch with a few bonuses that are cool. The DLC did, however, get really good reviews because it was fun but it'll cost more to get.

BDSP is also controversial because it's... a faithful remake? Okay, full disclaimer that I greatly dislike Diamond and Pearl, but these games are very similar to the originals so if you just want to play the DS games with a fresh coat of paint, this works fine. Most people are upset it wasn't more than just a remake, but it being just a 1x1 remake made Arceus possible, so it's a compromise. I'd recommend it if you want to play DP but more modern or a DS-era feeling game with modern mechanics. Otherwise, you may not enjoy it.

Legends Arceus is the least controversial Switch title; it only got flack for looking kind of ugly. It's a greatly fun and fresh experience with open zones you can go through (like Monster Hunter) and it's a very fun standalone title and all Pokemon can be caught. It has unique gameplay for battles and catching that no other game has, so it's absolutely a good option if you either like Sinnoh, want something new but still Pokemon, or just want a good Switch game that most people enjoy a lot.

And finally, SV. This is also controversial because it not only looks ugly but runs horribly, but honestly, I'd say it's one of the funnest titles in the entire franchise. It makes it very open for you to do whatever you want and has some really cool things it does that no other game has done and the multiplayer, while limited, is hilarious and made the experience a blast when playing with friends for the entire campaign. I'd suggest this if you like the idea of a game more like Arceus but also more classic in its structure because it is the best and worst of both worlds. Just note that it still runs poorly, though I would say the only real bugs I had were mostly visual (still ran terrible though).

Those are my thoughts; you have tons of options and honestly can just start anywhere, the nice thing is because of the variety you can just pick whatever you feel is funnest to you and most like what you want to play rather than what the internet thinks since the recent games have had a lot of frustration (not without reason though). Hope this writeup helps!