What computer program language proved to be the most useful and most profitable for you?

What computer program language proved to be the most useful and most profitable for you?

An interesting question came up in Quora and I just couldn't resist answering. The is "What computer program language proved to be the most useful and most profitable for you?" For me it's has been C#. For a more complete answer to this, you can check out my answer at Quora.