The single metric that best captures the core value a product delivers to users and correlates with long-term business success. Every team's work should be traceable to moving it.

Spotify: time spent listening. Airbnb: nights booked. Slack: messages sent per active user. A good north star metric reflects genuine user value (not revenue), is measurable frequently enough to guide decisions, and is not gameable by hollow optimization. It sits one level above revenue - if users get value, revenue follows.