Which Pokémon games were the main series’ first to use fully 3D polygonal graphics?

Pokémon X and Y were the main series’ first games to use fully 3D polygonal graphics.

Released for Nintendo 3DS in 2013, Pokémon X and Y replaced the long-running sprite-based presentation with polygonal character and Pokémon models throughout the game. The change affected the overworld, battles, animations, and camera presentation.

The games were also the first Pokémon main-series titles to receive a simultaneous worldwide release, arriving on 12 October 2013. They introduced the Kalos region, the Fairy type, Mega Evolution, character customization, and 72 new Pokémon species.

Earlier games used some 3D elements or special effects, and later games refined the approach further. That can cause confusion about which title first made the full transition. X and Y are specifically credited with being the first main-series installments presented in fully 3D polygonal graphics.