The choice depends entirely on your Pokémon's movepool. You should choose a careful nature Pokémon if your fighter relies on physical contact moves like Iron Head or Flare Blitz. Unlike the Calm nature, which lowers Attack, the Careful nature lowers Special Attack. This allows a physical powerhouse to become much tankier against Special attackers without losing any of its own punching power. It’s a staple nature for bulky physical threats like Scizor or Grimms.