Ask HN: Stack to start learning full-stack being a back end developer?

After working 3 years as a backend developer mainly with Java8, SQL, NoSQL, Docker and Jenkins writing webs and APIs, I want to keep improving as a whole developer.

I'd like to start learning more on the front-side of things so it'd help understand better my coworkers and understand better the whole picture of software, but I could use a little nudge to get into the right direction of what'd be best to learn.

Which stack's best as front to work with Java as back? Angular, React, just modern JS, another one...?

Thanks.