Answer: Pokémon Red and Blue
When you think about the classic Game Boy, you likely think about Tetris. Sweet, sweet, monochrome Tetris. And think about it you should because thanks to the practice of bundling Tetris with early Game Boy units, the addictive little puzzle game flooded the market to the tune of 35 million cartridges.
Outside of the boost provided by bundling, however, the best selling of the Game Boy and Game Boy Color platforms was Pokémon Red and Blue. Red and Blue, with their distinctive primary-colored cartridges, were the first wave of the Pokémon craze and collectively sold 31.38 million units.
While the sales numbers alone are impressive, consider this: the massive popularity of the Pokémon franchise and the runaway sales on the Game Boy marked the first time a Nintendo system’s best selling game wasn’t from the Mario franchise.
Source = “https://www.howtogeek.com/trivia/the-best-selling-non-bundled-game-boy-game-is/”