Ask HN: What language/framework should I learn for back end web dev in 2018?

I would like to invest some time and energy in mastering a web app framework (back end) for future projects. I have experience with ASP.NET, Rails, Node...but I wouldn't consider myself an expert at any of them. I would like to really dive deep and master a framework so that I can default to the chosen framework and "just build" things in the future. I'm primarily interested in producing RESTful or GraphQL APIs...so no need to talk about built-in views or which front end framework to choose.

I know there is no "perfect" answer to this question and some frameworks excel at certain things, but surely there are some frameworks which stand out for typical web application development in 2018. I definitely don't need to be on the bleeding edge of the web but I also don't want to invest my time learning a dying stack. (Or maybe there's no such thing as a dying stack?)

Popular opinions seem to be Ruby/Rails, Python/Django, and Elixir/Phoenix.

What would you invest your time in?