Double battles, abilities, natures — the competitive reboot.
Ruby, Sapphire, Emerald (and the Omega Ruby / Alpha Sapphire remakes) · Released 2002
Hoenn is the Gen 3 region, and the single biggest mechanics shake-up in Pokémon history. Ruby & Sapphire added abilities (Pokémon-specific passive effects), natures (stat-modifying personality traits), EVs (effort values — replacing Gen 1/2 stat experience), double battles, and weather teams. Almost every concept that shapes modern competitive Pokémon — Drought Ninetales, Sand Stream Tyranitar, Swift Swim rain — was invented in Hoenn.
Hoenn is based on Kyushu, Japan's southwest island. Its tropical climate, heavy rain and volcanic geography directly inspired the game's theme of weather and climate catastrophe (Kyogre causing endless rain, Groudon causing endless sun). The elevated land / sea / sky trio of Groudon / Kyogre / Rayquaza mirrors Japanese Shinto cosmology.
Ruby & Sapphire were the first Pokémon games to run on the Game Boy Advance's 32-bit hardware, which is why Gen 3 broke save compatibility with Gens 1 and 2 — the old games couldn't read the new Pokémon data structure, and there was no way to transfer mons forward until Pal Park in Gen 4.