Fairy type, Mega Evolution, full 3D — Pokémon's biggest visual leap.
X, Y · Released 2013
Kalos is the Gen 6 region, and the generation that finally moved Pokémon into full 3D graphics on the Nintendo 3DS. X & Y introduced Mega Evolution (temporary in-battle transformations that push Pokémon past their normal stat totals), the Fairy type (retroactively applied to 22 existing Pokémon), character customisation, and Pokémon-Amie affection mechanics. Kalos is also the smallest new-species generation since Gen 2 — only 72 new Pokémon, as development effort shifted to 3D model creation.
Kalos is based on France — Lumiose City is Paris (the Prism Tower is literally the Eiffel Tower), Coumarine City is the French Riviera, and Parfum Palace is Versailles. The legendary Xerneas (life) and Yveltal (destruction) reference the 3,000-year war lore that underpins the Kalos storyline, itself inspired by French Revolution themes.
Mega Evolution was originally conceived as a mechanic for every single Pokémon, but Game Freak scaled it back to a curated list after playtesting showed universal Megas would break game balance. Gen 6 shipped with 28 Megas; Gen 7 added 20 more; no new Megas have been added since.