At Cleveroad we have a fully-staffed Web development department that has built over a hundred apps with the Express.js framework. Why we choose Express.js when create a Node.js app?
First of all, the vast and active community of developers is important for both newcomers and experienced developers when building web apps with Node.js. Being able to get the professional advice is a great benefit. Of those three Node.js frameworks we are considering, Express.js has the biggest community. Just to compare, the framework has 7.9k followers on the Stack Overflow, while Sails.js has only 1k. Koa, unfortunately, trails behind.
Secondly, as for official documentation, Express.js docs are complete enough to help developers start off and provide support in tough cases.
The Node.js rest frameworks that utilize middleware approach do a better job in managing system resources. Express.js is one of them, which makes development and testing process more transparent. Moreover, as Express does not limit the choice of additional tools, we can freely handle with the integration of push notifications, payment gateways, in-app purchases, social media, analytics and everything our clients may require. (From this research - The best Node.js framework for your project: Express.js, Koa.js or Sails.js)