JavaScript. Trust me - it's set to grow immensely in enterprises over the coming years. Just read up a bit on Node.js - I'm currently building a project in it and it fortifies my love for JavaScript even more. While you're at it, look into the complete MEAN stack of technologies -
MongoDB,
Express,
Angular,
Node.js. The good thing about that stack? It's all JavaScript! So you'll be able to build a fully-fledged web application using just one language (and a bit of HTML and CSS knowledge).
I've been a C# engineer for more than a decade now and it's becoming a bit stale in my opinion. I've also been involved in Java for more than a year now and it's just more of the same. Seriously - the differences between C# and Java are few and far between. Learn the one stack and you'll easily be able to transition to the other. But Node.js is really something special... :love:
I wish there was a structured CPD (continuous professional development) process in software engineering that would force developers to continue learning. It would do wonders for industry and will make sure that you can appoint senior guys and make sure they're up to speed with current technologies and trends. I try and keep my team up to date with the latest happenings, hence me following the
hack.summit() 2016 the past 2 days. It really is very insightful hearing the presentations from the industry bigwigs.