Original: Pokémon Red and Pokémon Blue
To many, the original Pokémon games were nigh on perfect. Little did our 10-year-old brains know back then that the series would be improved so much over the years. Pokémon X and Y are certainly the culmination of that, but Gold and Silver began the trend.
The sequels to Pokémon Red and Blue were highly anticipated all over the world. They added a ton of new Pokémon to collect, a new region to explore, new mechanics, improved visuals (impressive for the Game Boy Color era), and tons more.
And a lot of that change was transformative: like the new dark- and steel-type Pokémon, Pokémon breeding, the ability for Pokémon to hold items to use in battle, fundamental improvements to the battle system, and an experience bar that showed the critters' progress to their next levels during battle. Those are all changes that have stuck around through six generations of games, and though every Pokémon game has improved on what came before it, Gold and Silver provided the biggest and best jump by far.