I have a few languages/technologies in my checklist, but the best thing that I learned in last six months is: How to be more data-driven & take better decisions.
As programmers we constantly look forward to learning new tools/technologies that will make our lives easier. However, I believe it's equally important to develop other skills that can help you as a programmer.
For example, if you are a developer and can build great stuff, you should learn a bit about marketing and how to sell in general. If someone is able to develop and sell to customers, they will probably want to learn about various analytics tools and methodologies. It gives you a new perspective and complements your existing programming skills.
My $0.02 :)