I pulled the trigger at work that all new services we build in Node use Koa if the need a web framework because it utilizes ES6/7 features most effectively while still having a good bit of simplicity. That said, all of our legacy services, in Node, that need a web framework use Express. I find Express is a good beginner web framework and then once you understand ES 6/7 features in more depth, Koa becomes rather easy to switch to.