Ask HN: Best programming language to learn for expanding my knowledge?

Hi to all, I am a Ruby developer who primarly works on the web. I know also Javascript, and just enough PHP to be dangerous.

But I feel that I need to learn a most 'complete' or better, lower level, programming language to improve my arsenal of weapons and to improve the way I think while programming.

I am fascinated about a lot of languages, just to be clear, I am talking about the ivy league, something that is where C++ or Java are. Any advice?

Which is the best choice to improve my skills, and that can help me make the differences where languages like Ruby or Javascript can't?