I guess I should begin my time here with a classic: what's everyone's favorite Pokemon game(s)?
Mine is Pokemon HGSS (the amount of content is mindboggling), Pokemon Platinum (grandest Pokemon experience) and Pokemon Black 2/White 2 (most fun and unique playthrough). I'm also looking forward to buying Legends: Arceus on the Switch, have heard many great things about it!
Honestly I'd probably have to say either USUM or BDSP. Their post-games were enjoyable for me. SV are too unpolished to make me want to keep playing it.
This has always been a difficult question for me. The newer games have so many quality of life additions that I find it harder to go back to the older titles... but then, the older titles tend to be more polished and have more "charm" to them.
I also mostly play in the post-game, not replaying the story.