Well lets see I know Basic, Visual Basic, Fortran, Pascal, C, C++, Java, Perl, JavaScript, Winbatch, C#, Various shell languages, and I'm probably forgetting a few.
If I could only learn one it would be C++. The reasons?
1) Anything and everything is and can be done in C++.
2) It is reconized as the lanugae of professionals so companies are will to pay more for the skill.
3) Most of the other languages are written in it (the compiler, and tools)
4) It provides the fastest programs and most efficient programs.
My second would be Java, for most of the same reasons.
On the other hand if I was asked what lanuage I thought was the lanuage that I could learn the fastest, and do the most with in the shortest period of time. I guess I would have to go with Visual Basic. The original Basic was created to teach Fortran and has a easy to understand syntax, and Microsoft has really used it as their standard scripting lanuage, and as such it has tons of supporting libraries and such so you can do all kinds of things, it is the script language of all the office tools, it and its kissing cousing VBScript can be used in Web applications.
You can say that Microsoft has a sweet spot for Basic. It was their first lanuage, the one these sold before IBM wanted an OS to go with it. This was long before Visual Basic, but it is what started Microsoft.
If I was asked to pick a lanuage that could do almost anything, but wasn't for GUI's it would be Perl.
I can put together a Perl program to do almost anything especialy parse files, in less code and faster then any other lanuage I know.
BTW if I was more into Web I might get into PHP it seems to be pretty good for that.