Which Pokémon generation introduced the most new species?

Generation V introduced the most new Pokémon species, with 156.

The generation debuted in Pokémon Black and White for Nintendo DS in 2010. Its setting was the Unova region, and the games introduced species ranging from Victini, National Pokédex number 494, to Genesect, number 649.

Generation V's 156-species total is larger than every other generation's initial group. Generation III added 135, Generation II added 100, Generation IV added 107, and Generation VI added 72. Black and White also took an unusual approach by focusing the main story's regional Pokédex on new Pokémon before the post-game.

The figure refers to new species introduced by the generation, not every Pokémon obtainable in Black and White. Later games and forms can change what is available, but they do not alter the fact that Generation V added the largest single generation-level group.