This is mostly dependent on which KPIs you are going to view and which data is most important to you. In an ecommerce application, this can be number of views for particular items, number of drop-offs or page bounces and exits. In a weather app, this can be number of cities added or whether user selects Fahrenheit or Celsius degrees. Depending on the events, you need to implement custom events, explained in each SDK documentation.
If you want to generate mobile or web code easily for custom events, user profiles, crash reporting and all other features that comes with Countly in general, we suggest that you use Countly Code Generator, which is a point and click service that builds necessary code for you.