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Gen 2. It introduced the most core and fundamental additions.
>Day/Night cycle
>Berries
>Hold Items
>Breeding & Eggs
>Radio/Gear
>EXP. Bar
>Moves working correctly
>Dark & Steel types
>Spc ATT & Spc DEF
..and a lot more subtle things we take for granted today.
Gen 3:
>Natures, Abilities, EVs
>Double Battles
..while removing day/night cycle and headbutt trees. Emerald's animated sprites (soulless) are also worse than Crystal's (soul)
Gen 4:
>Special and Physical split
>3D elements
>Wi-Fi online