What is the best way to export an object literal with ES6/2015?

You can export object itself:

export default {
    googleClientID:'xxxx'
};

The difference is that in your case you will get brand new object every time you call exported function. In this case you will get the same object every time. Depends on what you need.

You can simply export an object

export default { googleClientID:'xxxx' };

A default export can be a function, a class, an object or anything else. This value is to be considered as the "main" exported value since it will be the simplest to import.