I didn't have an answer for a very long time. Or more seriously, the answer was "the one that paid the most". I've run the gamut of popular languages, C, C++, Java, Javascript, perl, ruby, Python, Visual Basic, VB.Net, C# and F#.
But the last couple years it's really been C#. The pace of development on the language/runtime has really picked up with yearly releases. The features that are added and iterated on are expressive and intuitive. You can tell from the discussion posts on how a feature is being considered for inclusion is thoughtful and deliberate. It really feels like the language is in good hands.
Just wish those hands weren't Microsoft.
Everything is in the link.