Legends arceus is the best pokemon experience in my opinion as someone who's played all games and most spinoffs. However, the other switch games don't stand out as much (Let's go is fun for gen 1 nostalgia but the diamond/pearl remake is very basic) so a 3DS is better "value". That being said, only get a 3DS if you wanna mod it to pirate the games because used pokemon prices are completely insane these days and its nintendo so ¯\(ツ)/¯
You can also play legends arceus and the other Switch games on PC via emulator. Same with 3DS but the actual hardware is a much nicer experience for obvious reasons. If you go with Switch then I suggest Arceus and Let's Go and if you go with 3DS I would suggest Black/White (and BW2 if you enjoy the first game) and, Omega Ruby/Alpha Sapphire and Ultra Sun/Moon (they also all have ROM hacks for difficulty/extra mons).