Which Pokémon game was the first main-series title to use fully 3D graphics?

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

Released worldwide for the Nintendo 3DS in 2013, the games presented environments, characters, battles, and Pokémon models in three-dimensional form. They also introduced the Kalos region, Fairy-type Pokémon, Mega Evolution, and character customization.

Earlier games had already used 3D elements. Pokémon Stadium and Pokémon Colosseum were fully 3D spin-offs, while Pokémon Diamond and Pearl used a mixture of two-dimensional sprites and three-dimensional environments. The distinction matters: X and Y were the first core role-playing games to make 3D presentation their standard visual format.