My background is in Macromedia Flash but I've decided to branch out and learn a more "serious" language. So, my question is this:

From the standpoint of industry demand, what is the best language for me to learn right now?

I'm looking at C++/Java/VB.net/C# as my inital possible choices. Keep in mind that I want a language that is powerful and in demand but I also want something that will teach good programming practices and will work as a "springboard" for learning other languages.

Thanks for the input.