>>23371448Gen 2 did the most. The other gens added sorely needed mechanics, but you have to look at how different gen 2 was from gen 1. The list is long, so buckle up. Pic related.
>Sp Attack and Sp DefenseAbsolutely integral to the modern gameplay style. It completely redefined how certain pokemon were used, for better or worse.
>Held itemsProbably the biggest change, held items added a completely new element to battle and a simple new mechanism for evolving pokemon. Held items are still absolutely critical to competitive play, and later went on to even allow changes to breeding mechanics.
>Time cycleCompletely changed how to go about catching all the pokemon, as certain mons appeared only at certain times of the day/week. Certain characters were only intractable at certain times. Mechanic even saw reuse in later gens even after being dropped in gen 3.
>BreedingBasically introduced competitive pokemon. It was now possible to breed pokemon with superior stats and egg moves.
>Move Deleter/TutorYou could finally take HMs off pokemon. I think everyone appreciates this one. Gen 2 also saw the first Move Tutor, a guy that visited the game corner on specific days after you beat the Elite Four in Crystal would teach your pokemon moves (without just giving you a TM) for coins.
>Rebattling trainersYou could finally rebattle people outside the Elite 4. Huge boost for replayability, and a way to allow access to special pokemon (hordes) or items.
>Mystery GiftReally the first way for the player to get special gifts outside of just playing the game normally. In later gens, this turned into a way of distributing event-pokemon and items.
>Decorating Player's roomFirst instance of player customization. Preceded stuff like Secret Bases or Clothing Customization.
>Dark and Steel typesRedefined the game balance for several years, and introduced two groups of completely different pokemon.
>ShiniesIcing on the cake, really, but this ended up becoming crazy popular.