It's really, really hard to give a good answer for that. I have a handful of languages that I've put intense study in: C++, Delphi, ColdFusion, ASP, ASP.NET and SQL being the big ones. Most of those I invested energy in because I used them in my job. C++ is just a really good language for getting a deep understanding of many programming concepts that carry over to other languages well.
For me, esoteric languages are the answer. Things like Lisp (or any dialect), Haskell, Erlang, etc will challenge your assumptions about how program flow should work, what makes a "correct" program, etc. They will influence how you think about the languages you already know. For example, study a functional language, and then think about concepts like TDD/eXtreme Programming/etc. Suddenly, your notion of what makes an appropriate function will radically change.
I also pushed away from learning languages towards programming strategies. Study design patterns, algorithms and data structures, compiler theory, software development lifecycles, etc. Those are the things that will make you a better developer in EVERY language. I try to be a specialist in the languages that make me money, but also be as good a programmer as possible across all languages. To do the latter, I have to stop thinking about individual languages.