There are 25 Natures that a Pokemon can have in Brilliant Diamond and Shining Pearl. Of those, five have absolutely no effect on the Pokemon and its stats. Essentially, those five Natures are useless in Brilliant Diamond and Shining Pearl.
Other Natures will boost one stat and lower another. Truthfully, a Pokemon should go with a Nature specific to its stats, but some are more common and popular than others.
Some of the best Natures in Pokemon Brilliant Diamond and Shining Pearl
The Timid Nature in Pokemon Brilliant Diamond and Shining Pearl raises the Speed stat and lowers the Attack stat. This should be used for speedy Special Attackers, leaving its defenses in place and lowering its unused Physical Attack stat. This is very helpful against some of the physical defenders in the game.
Calm increases the Special Defense and decreases the Attack of the Pokemon with the Nature. Physical Defense is in abundance in Brilliant Diamond and Shining Pearl. So, you might want to focus on a Special Defense creature. This is good for one who doesn't use Physical Attacks.
The Careful Nature in Pokemon Brilliant Diamond and Shining Pearl is the counterpart to Calm. Special Defense goes up and Special Attack goes down. Your Special Defense partner with a Careful Nature should have its Attack stat be its primary focus of offense.
Adamant will lower a Pokemon's Special Attack and raise its Attack. This is great for a Physical attacker who doesn't want to lose any of its Speed or Defense stats. Special Attack won't be used as it slashes away at its opponents with Physical strikes.
Physical defenders are everywhere in Brilliant Diamond and Shining Pearl. A Pokemon with the Modest Nature will see a higher Special Attack and lower Attack. Since its Physical Attack isn't necessary, the Special Attack stat will decimate the physical defenders.
Note: This article is subjective and reflects the opinion of the writer.