I am looking for a tool using which I can do automated testing of REST APIs. I was going through SOAPUI. But as our clients prefer Microsoft products, I would like to know if there are any Microsoft API Automation tools. Any help will be great!
Visual Studio Load Testing with Web Tests does what you are asking for. You can use the GUI interface to create web tests, or convert them to coded web tests if they are too complex, or you just prefer that. http://msdn.microsoft.com/en-us/library/vstudio/ms182594.aspx
The biggest hurdle to using Visual Studio Load Testing is that you need to have a Visual Studio Ultimate license which is relatively expensive.
The answer will depend on the type of testing you are doing, but you should just use Visual Studio and write some code to call the API directly. You can also=use the unit test infrastructure it provides, or nunit to write your tests.
The best tool to test an api is code that calls it.