What is the best AI tool for writing unit tests?

GitHub Copilot and Cursor both excel at generating unit tests, often producing better tests than developers write manually because they're thorough about edge cases. You can ask Copilot Chat to "write comprehensive unit tests for this function" or use Cursor's agent mode to generate a full test file. Tests are one of the highest-ROI use cases for AI coding tools — the quality is high and the time savings are significant.