Pokémon Legends: Arceus is the latest entry in the Pokémon series and takes a different approach to gameplay and style. What remains true is the Pokémon, and every trainer's first decision is to choose the starting Pokémon.
In Pokémon Legends: Arceus, players can choose between Cyndaquil, Oshawotta, and Rowlet. As beginners often stay in a player's party throughout the game, getting the best nature is key to making them more powerful. Here are the best natures for Cyndaquil, Oshawott, and Rowlet.
Related: What does "arceus" mean? Origin of the name of Pokémon Legends Arceus
Cyndaquil is a fire-type Pokémon that has a high special attack and speed. Because of this, you'll want to increase his speed so he can outrun the opponent and get the first hit. This makes the best nature for Cyndaquil the shy nature. This will lower his attack and increase his speed.
Oshawott is a Water-type Pokémon with a fairly even Attack and Special Attack. This makes it somewhat unpredictable as you can focus on either form of attack. This makes the best nature for Oshawott the inflexible or modest nature. Adamant will increase his attack while Modest will increase his special attack. Choose the nature you prefer to focus on.
Rowlet is a Grass-type Pokémon with a fairly equal Attack and Special Attack and good Special Defense. This makes it an offensive or defensive Pokémon. The best nature for Rowlet is either the calm or shy nature. Timid increases his speed, allowing him to fire the first shot more reliably. Calm will increase his special defense to survive more hits. Your choice will reflect how you want to use Rowlet.
For more information on Pokémon, see Pokémon Go Raid Schedule: Bosses and Counters (January 2022) and Are there trainer encounters in Pokémon Legends: Arceus? on Pro Game Guides.