Choosing the right unit testing framework for .NET Core projects can be challenging, with MSTest, NUnit, and xUnit.Net being the three primary options. Each framework uses attributes to define test methods and execution flow, but they differ significantly in their approach to test isolation, extensibility, initialization, and parallel test execution. NUnit, popular for its ease of use and wide platform support, is often chosen for Test-Driven Development (TDD) in C#. xUnit, created by the original authors of NUnit to address its limitations, emphasizes test isolation and extensibility, making it a robust choice for Selenium automation testing. MSTest, integrated with Visual Studio and now open-source, offers a straightforward approach with features like parallel test execution at both method and class levels. Each framework has its strengths, with xUnit standing out for its community-driven improvements and extensibility, making it suitable for projects requiring clean, maintainable test code.