But as you probably want to slightly more concrete list I would go with JavaScript, Java, Clojure, Objective C, Ruby, SQL, Prolog. In that list we have a few of oo languages of varing purity, a few scripting languages, a couple of logic programming langs, and a lisp. Something for the enterprise, something for the academics, and something for the startups. Something for the web, something for server and something for your desktop and phone. You'll end up well rounded, and have a tool for most situations.
But as you probably want to slightly more concrete list I would go with JavaScript, Java, Clojure, Objective C, Ruby, SQL, Prolog. In that list we have a few of oo languages of varing purity, a few scripting languages, a couple of logic programming langs, and a lisp. Something for the enterprise, something for the academics, and something for the startups. Something for the web, something for server and something for your desktop and phone. You'll end up well rounded, and have a tool for most situations.