Which breathing exercise is best for panic attacks?

Extended exhale breathing, not box breathing. During a panic attack, your sympathetic nervous system is in overdrive. You need to activate the parasympathetic brake as directly as possible. Inhale for 4 counts through your nose, exhale for 6 to 8 counts through your nose. The longer exhale stimulates the vagus nerve and begins slowing your heart rate within 60 to 90 seconds. Box breathing (4-4-4-4) is often recommended, but its equal inhale-to-exhale ratio doesn’t provide the same parasympathetic push. Save box breathing for when you’re already relatively calm.