Honestly I'd say JavaScript.
JavaScript isn't just transitions, alert boxes and form magic. It's a versatile language found in the browser, on the server and on hardware. Build robots, make websites, spin up microservices.
Most importantly it's the friendliest and fastest growing community.
It looks like you're more at home in a backend environment, so take a look at Node.js.
Good luck!
If you really don't want to learn JS, take a look at Ruby.