Quiet Nature

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

Competitively, Regice, Tangrowth, Swampert, and Reuniclus run Quiet most often. The +Sp. Attack boost complements their role, and the -Speed penalty barely affects their game plan.

For the right Pokemon, Quiet 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 Quiet Mint from the Battle Tower changes the stat effects to Quiet.

Short answer: Quiet for Trick Room power, Modest 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.