Pick the single metric that most directly reflects the value customers are getting, and that reliably predicts revenue a few steps downstream — for a subscription product that's often weekly or monthly active usage of the core feature; for a marketplace it's often completed transactions; for a services business it might be qualified pipeline generated. Avoid picking a vanity metric like signups or pageviews just because it's easy to move — it should be genuinely hard to fake and hard to improve without also improving the business.
Test the choice by asking whether every team could plausibly rally around it weekly — if it only makes sense to marketing, it's the wrong metric.