Here are some popular choices, along with reasons why:
Early Game:
* Dialga: It's the legendary Pokemon that's central to the story of Diamond. Catching it early will give you a powerful advantage.
* Giratina: If you're going after the "original" Pokemon, this is your best option for a strong early-game partner.
Mid/Late Game:
* Shaymin (Sky Form): This rare legendary Pokemon is very powerful, and its Sky Form makes it a versatile attacker.
* Manaphy: This mythical Pokemon is known for its powerful Water moves, and it can be very useful in later battles.
Other Considerations:
* Rare Pokemon: If you're a completionist, you might use the Master Ball on a rare Pokemon like a shiny or a Pokemon with a very desirable nature.
* Personal Preference: Ultimately, the best way to use the Master Ball is to use it on the Pokemon that you want to have on your team the most!
Important Note: In Pokemon Diamond, the Master Ball is only available from the game's developers. It's not available in the game itself.
Remember, the Master Ball is a one-time use item, so choose wisely!