There's a lot that one can test in any application. How can you figure out what to focus on with so many options?

Summary

A good rule of thumb is always to tackle the business needs first.

When you know what’s most valuable for the business and the organization, the time will come to begin the implementation portion of your work. Your strategy depends on your project and the surrounding situation. However, a safe bet is to go deep with easier-to-maintain tests while sprinkling in more extended end-to-end tests, so long-term maintenance issues do not bog you down.