I voted express but Koa v2 or hapi are good options too.

Koa v2 as well

Another vote for Koa!

If I want it quick, Then I'll use Loopback.

Pros

• Very quick RESTful API development
• Convention over configuration
• Built in models ready to use
• RPC support
• Fully configurable when needed
• Extensive documentation
• Fulltime team working on the project
• Available commercial support

Cons

• Learning curve can be pretty steep because there are so many moving parts

Koa v2

For a quick prototype, definitely Meteor. Its developer experience is very, very good. Making it extremely easy to quickly create a functional prototype. As long as you don't have to host it anywhere, that'd be my pick.

If the MVP would have to be hosted somewhere, I'd go with the Node stack (either Express or Sails) because it's way easier to setup.