This is totally up to you. However, if you are doing ".NET" development, not knowing C# would be just ridiculous.
From this standpoint, "what's the next" does not seem to be correct. The correct answer would be "what's the recommended first .NET language to learn?" :-) This is because VB.NET, even if it can be considered as a decent language by its properties, still cannot be considered as a first-class citizen, as it hardly will be standardized, while C# and C++/CLI are in ECMA and ISO standards.
At to everything else, you really need to decide by yourself. We don't know your goals, background and skills, so the answer by some strangers hardly can be something to depend on. I would say only one thing: if you want to do at least professional development, maybe even build a career in the fields, your language repertoire should be not one or too language, but at least a few. I would advise to learn about languages in general, read some books and articles on language theory, history and practice before you can come to a decision. Another advice: try many, just a bit of each, but to some non-trivial point in your learning, and read the overviews.
Please start with:
http://en.wikipedia.org/wiki/Programming_language[
^].
A language that doesn't affect the way you think about programming, is not worth knowing.
And every developers needs to read this, first of all:
Peter Norvig, Teach Yourself Programming in Ten Years,
http://norvig.com/21-days.html[
^].
—SA
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