Which Pokémon game was the first in the series to feature 3D battles?

Pokémon Stadium was the first Pokémon game in the series to feature 3D battles.

Nintendo released Pokémon Stadium in Japan for the Nintendo 64 on August 1, 1998. The game presented battles with fully polygonal Pokémon models, replacing the two-dimensional battle visuals used by the original Game Boy games. It also included tournament cups and a Free Battle mode.

The Japanese release initially supported a limited selection of Pokémon. The international version, released in 2000, expanded the roster and allowed players to use Pokémon from Pokémon Red, Blue, and Yellow through the Transfer Pak accessory.

Pokémon Stadium is sometimes confused with Pokémon Colosseum or Pokémon Battle Revolution because those later console games also emphasized 3D combat. Stadium came first and helped demonstrate how the handheld series could translate into a home-console battle experience.