What was the first Pokémon game series to feature speaking player-controlled Pokémon?

Pokémon Mystery Dungeon was the first Pokémon game series to feature speaking player-controlled Pokémon.

The series began with Pokémon Mystery Dungeon: Red Rescue Team and Blue Rescue Team, released in Japan in 2005. Unlike the main role-playing games, the player character is transformed into a Pokémon and explores randomly generated dungeons.

The games are spin-offs developed by Chunsoft, now known as Spike Chunsoft. Their gameplay combines Pokémon battles and recruitment with the turn-based dungeon-crawling structure associated with the Mystery Dungeon franchise.

Pokémon in these games can communicate in human language, form rescue teams, accept missions, and recruit other Pokémon. Red Rescue Team was released for Game Boy Advance, while Blue Rescue Team appeared on Nintendo DS. The pair reached international markets in 2006.