It depends on your goals. If your goal is to do a startup, then the obvious choice is to really master an entire web stack. For instance linux / nginx / python / html / css / javascript. If you want to do research in computer graphics, then you better learn OpenGL and C++ or .NET. Want to work in embedded systems? C. Scott's recommendation of C# & Javascript is a solid and pragmatic choice, good for those looking for…
Agreed too for your opinion about how to do a startup. But I want to add C to your full stack suggestion. Sometimes it pays off to know low-level details about your choice of technologies. Almost every important technologies that I know is implemented in C.
In order to be a well rounded developer you ideally have familiarity with functional programming, declarative programming, lisp, and a much more. You hit diminishing returns pretty quickly though.