Hi Jon Snow, while Express is easy to get started with, I'd suggest Hapi!

I would say Koa. It will make your life a lot easier.

There's no such thing as a "REST APIs framework that plugs well with EmberJS".

EmberJS can consume a REST API. You won't have to worry about the framework internals as long as it adheres to the standard. So, every REST API framework will be exactly the same from this perspective.

I would say express :) Only because I haven't used Hapi yet. Also created a rest api starter project on github from some great tutorials. Check it out: https://github.com/ayoungh/Rest-API