I like jquery-ui. It has some nice themes buildt in, and many plugins available. But most important: it's very leightweight!
Since jquery is the "native" javascript plugin in rails, jquery-ui should also be the best choice. There is even a nice plugins for rails
= 3.1, which configures all you need for the asset-pipeline:
https://rubygems.org/gems/jquery-ui-rails
If you need some "special" plugins, eg for a MVVM approach, just look for nice plugins, ideally with themeroller-support. Example:
http://datatables.net/ , and many more.
If you want to have a bad time, consider using heavyweight and slow JS-Frameworks like ExtJS (Sencha). Implementing the UI then would take way longer then the whole application without styling, only for the benefit of having some complex widgets. Exspecially ExtJS development feels exactly like raw Java/Swing. Your choice.