I believe that the most important metric for Google Photos would be user engagement. This includes metrics such as the number of photos uploaded, the frequency of app usage, and the amount of time spent using the app. User engagement is critical to the success of any product, as it indicates not only that users find the product useful, but also that they enjoy using it. Additionally, high user engagement can lead to increased user retention, as well as opportunities for monetization through advertising or premium features. However, it's important to note that user engagement should not be the only metric tracked. Other important metrics could include user satisfaction, customer acquisition, and revenue growth. By tracking a variety of metrics and using them to inform product decisions, we can ensure that Google Photos continues to meet the needs and expectations of its users while also driving business success.