156 new Pokémon, zero returning — the biggest roster in franchise history.
Black, White, Black 2, White 2 · Released 2010
Unova is the only Pokémon region to ship with an entirely new roster of Pokémon — Gen 5's Black & White deliberately excluded every single Gen 1-4 Pokémon from the main storyline, forcing players to build a team entirely from 156 new creatures. This was a deliberate design choice by director Junichi Masuda, intended to evoke the 'everything is new' feeling of the original Red & Blue. It is also the largest single-generation roster in franchise history.
Unova is the first Pokémon region based on a location outside Japan — it's modelled on New York City, with Castelia City as Manhattan, Driftveil as Chicago and Nimbasa as Coney Island. The bridges that dominate the map's skyline reference the real-world East River crossings.
Black 2 & White 2 are the only direct sequels in mainline Pokémon history — every other 'third version' (Crystal, Emerald, Platinum) was a remix of an existing story, but Black 2 / White 2 take place two years after the original B/W events with a new protagonist.