If we're unable to transfer our Pokémon first, probably Scarlet since most of my most cherished Shinies are on that game atm. Which is kind of a bummer since I can't play it while multitasking.
If we got one final chance to transfer before dropping everything else, probably Sword. Not my fave, but most of my most cherished 'mon can be transferred in and I can play it while multitasking, unlike SV and Arceus. Plenty of Shinies to hunt too and it's easier to spot out those 1/65536's. ORAS if we were somehow able to transfer our Pokémon to older gens, at least as far back as the game they were originally caught in, so I could use Mothim too (no HGSS since my Flareon was caught in X).
In the event that we were forced to start from scratch, probably Crystal. Smaller roster than most options, but I adore the Gen. II aesthetic and find those early animations really neat (especially since I experienced the static sprites of Silver as a kid). Virtual Console version so we can avoid that whole dry battery thing.