NUnit, xUnit, and MSTest are three popular unit testing frameworks in ASP.NET Core, each with its own strengths and weaknesses. NUnit is a free, open-source framework that works well with Selenium and web drivers for cross-browser testing. It has a simple attribute-based syntax and is easy to use, making it a good choice for developers who want a straightforward unit testing experience. xUnit, on the other hand, is a community-focused, free, and open-source framework that offers more advanced features, such as data-driven testing and parallel test execution. It also has a more modern and flexible syntax, which can be beneficial for developers who want to write more complex tests. MSTest, developed by Microsoft, is also a popular choice, offering features like cross-platform support, data-driven testing, and parallel test execution. However, it requires more setup and configuration compared to NUnit and xUnit. Ultimately, the choice between these frameworks depends on personal preference and specific project requirements.