At a bare minimum, we use ES6/ES2015 standards across all of our applications. Whether we need transpiling or not depends on the environment we are building for.
Node server-side applications
User interface web applications
For user interfaces that are presented to the web browser, we use webpack and babel to transpile our application into browser-friendly ES5. Generally we want to keep the version of ECMAScript in sync with our Node.js version, so we currently support ES2015.