Since its inception in the late '90s, Pokémon has been one of the most popular media franchises of all time, and a huge portion of that success is thanks to the games. Pokémon games have defined and pushed boundaries within the role-playing genre, with their emphasis on collecting and battling with monsters inspiring a whole host of games that have followed suit. Pokémon is now at a roster over 1000 strong, released across nine generations, with at least two games released each time.
Source：https://www.cbr.com/pokemon-games-gens-difficulty-ranked/
