Brave Nature

Brave gives your Pokemon 10% more Attack at the cost of Speed. That tradeoff works best when the lowered stat isn't part of your moveset. Pokemon with Brave nature prefer Spicy flavor and dislike Sweet.

Competitively, Bronzong, Melmetal, Rhyperior, and Aggron run Brave most often. The +Attack boost complements their role, and the -Speed penalty barely affects their game plan.

For the right Pokemon, Brave is one of the best natures you can pick. The -Speed drop actually helps under Trick Room, where slower Pokemon move first. Just don't slap it on something that uses Speed-based moves.

Breeding with an Everstone is the most reliable approach since Gen 5. The parent holding it passes its nature to every offspring. Synchronize works for wild catches. And if you already have the Pokemon, a Brave Mint from the Battle Tower changes the stat effects to Brave.

Short answer: Brave for Trick Room power, Adamant for standard power. There's no universal best. Check your Pokemon's base stats and common threats in the format you're playing to decide which tradeoff is better.