Pokémon Ruby / Sapphire

Ruby and Sapphire reinvented Pokémon for GBA with 135 new creatures, the Ability system (every Pokémon with a unique trait), Natures affecting stats, and double battles. Hoenn was the most varied region: volcanoes, oceans, underwater caves. Secret Bases let you create personalized hideouts. The controversial cut of previous-gen compatibility was contentious but necessary for the technical leap.