Angular 2 - https://angular.io/ is already written in ECMAScript 6 and includes mobile features support for touch event gestures, tuning for performance and low-memory usage on mobile platforms, UI components with responsive, cross-device support, animation support and I18N.
Ember allows developers to create scalable single-page applications but also comes with a lot of built-in support for standard code features like a rock-solid behavior when the user clicks the browser Back button. Ember is a better choice if you’re creating a multipage, long-term project.
Backbone.js is a lightweight MVC framework, which means it’s good for building fast and responsive applications—especially single-page web apps.
In addition to frameworks there are a couple other thinks to look for depending on the other software technologies you want to use.
The best way to select an outsourcing partner is ask which of these frameworks, languages and stacks they recommend for your specific application when you are hiring a partner to develop your entire app. Then weigh their answers along with references to clients where the partner has used the recommended technology.
However if you are looking to extend your existing development team with an outsourcing partner then you should ask for the specific framework experience you need to fit the decisions you have already made. Ask for evidence of hiring the type of developer you need and ask to interview candidates yourself. That way you will get developers that fit your technical requirements as well as your company culture.