XY. Or rather, best dex+catching. There's good dexs that have great mons that you can only find in Victory Road, or ones where you have 1 option before Gym 3 and have to run that every time. Shoutout to SwSh with an incredible dex entirely locked behind the raids and dens, making finding something specific a serious chore. XY is not only broad, a lot of the diversity is available early. That makes monotype challenge runs, nuzlockes, etc, all varied and reasonably simple.
SM...honestly a good dex, but the prevalence of derps in early routes makes actually recognizing that fact pretty difficult.
SV is probably a contender, since sequence breaking for anything in the game is a reasonable option, but I'd need more experience with it to vote for it.