Careful Nature

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

Competitively, Ting-Lu, Alomomola, Ferrothorn, and Gliscor run Careful most often. The +Sp. Defense boost complements their role, and the -Sp. Attack penalty barely affects their game plan.

For the right Pokemon, Careful is one of the best natures you can pick. It fills a solid niche for defensive builds. Just don't slap it on something that uses Sp. Attack-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 Careful Mint from the Battle Tower changes the stat effects to Careful.

Short answer: Careful for physical wall utility, Calm for special wall utility. 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.