Ask HN: Which MVC JS Framework is Best?

About to start a new heavy client-side interaction project, and I want to start it with the right structure so I can move fast and easily maintain it.

I've been looking at the following libraries :

Spine.js : http://spinejs.com

Backbone.js : http://documentcloud.github.com/backbone

Knockout.js : http://knockoutjs.com

Batman.js : http://batmanjs.org

So far, I'm leaning towards Batman, though I've used Backbone in the past and enjoyed it.

Has anyone worked with these? What are your thoughts? Why do you like one over the other?

Thanks!