xUnit.NET is free and open-source, this unit testing tool for the .NET Framework is written by the original creator of NUnit v2.
Ranked in these QuestionsQuestion Ranking
Pro Supported and used by Microsoft itself
.NET Core area etc.
xUnit.NET has an impressive list of extensions which other testing frameworks are lacking.
Pro More intuitive than other testing frameworks
xUnit.NET uses intuitive terminology and is frequently updated to be more lean and user-friendly.
Con Lack of documentation
Compared to MSTest and NUnit, xUnit.NET lacks documentation.
Con .NET 4.5+ only
Isn't available prior to .NET 4.5.
(As of today 2018, this isn't really a disadvantage)