Depends on your past experience and programming background. If you're coming from Delphi or Java, get C#/ASP.NET.
I am a .NET developer transitioning to PHP and find the .NET server platform and tools tightly integrated and easy to use. On the other hand, it runs on Windows ONLY and the professional tools are cost prohibitive. I selected PHP, because it is the standard scripting language and the LAMP stack and open-source offerings are plenty. Good luck!