There is such a huge list to choose from. Do you have any idea what you want to do with it once you've learned it? If you're going for purely academic value, I'd go with haskell. For practicality, I'd say a good foundation in C is never a bad start. If you don't have longterm aspirations and just want to be able to "do" quickly, perl or python might be good choices.
The more information you can give about your intentions, the better advice we give.
-Lee
Edit: After your last post, I'd say a C-style language will serve you best. Most games use C or C++ under the hood. Most OS X apps are Objective-C which is a proper superset of C. Python and Ruby both have Obj-C bridges so they can be used for OS X app development, too. For serious math programming, fortran 95 might be a good choice.