Master Ball

The Master Ball is a type of Poké Ball that can unfailingly catch a Pokémon. It has not been implemented into Pokémon GO currently, however code does exist in the game and can be obtained via and running private servers.

In-game description
''The best Poké Ball with the ultimate level of performance. With it, you will catch any wild Pokémon without fail.''

Trivia

 * Mewtwo cannot be caught without a Master Ball in the original Gameboy series.