I think NUnit is your best bet. With TestDriven.NET, you get great integration within Visual Studio. (ReSharper also has a unit test runner if you’re using it). NUnit is simple to use and follows an established paradigm. You’ll also find plenty of projects, tutorials, and guides using it which always helps.
Your other main choice is probably MbUnit, which is more and more positioning itself as the BDD framework of choice (in conjunction with Gallio).