It doesn't matter. Pin a bunch of names on a dart board, throw a dart, and pick the one closest to the dart. Seriously, it really just doesn't matter. Not even a little bit. At least not in terms of whether or not your side project succeeds. You will NOT succeed or fail based on whether you chose Node over Ruby, or Java over Erlang, or Angular over React, or PostgreSQL over MySQL, or Mongo over CouchDB, etc., etc., yada, yada, yada.<p>Now if the question you're really asking is "which one should I learn from a long-term career standpoint?" or something along those lines, the equation changes a little bit. BUT even there, the simple reality is, you can be gainfully employed, making good money, doing fun work, using any technology you listed, and plenty you didn't.<p>If you're coming at it from the career / long-term perspective, and want an objective measure, then scan the job boards, and see what you find the most open reqs for. Go with that. Otherwise, pick two or three off your list, dabble in each, and see which one feels right to you.<p>The only thing I'd explicitly advice against is going with something really outside the (modern) mainstream, like COBOL, or some really new'ish languages which may or may not catch on (Nim, something like that), or any "esoteric" languages: Brainfuck, Intercal, Whitespace, Befunge, etc.<p>As to what <i>I</i> personally would pick? Groovy and Grails, along with PostgreSQL, and one of the popular JS frameworks for front-end (React, Vue, Angular, etc.). But that's just down to familiarity and personal preference.<p>Edit: let me add one bit of clarification... I don't mean to slag Nim, or any of the other newer'ish languages. And I don't mean "don't learn it" in the general sense. I just wouldn't "bet the farm" one of those just yet. By all means, dabble with lots of different languages, and constantly pay attention to how they are evolving over time, and how they are (or are not) gaining industry acceptance.