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I would say HGSS
>Kanto is mostly restored
>Safari Zone updates with post-Gen 2 mons
>Bug Contest updates with post-Gen 2 bugs
>Calling with Gym Leaders on their days off for rematches
>Trade with various characters like Steven, Jasmine or Brock
>Battle Tower and the rest of Sinnoh's Battle Frontier
>Latios/Latias becomes a roamer
>Embedded Tower for Kyogre/Groudon
Also I am really going to miss seeing Gym Leaders use modernized teams now that the dexcut is a ting.