What is your best rich web app development tool?

I am interested to know your best tool for rapid yet maintainable rich web app development. I know few tools and would like to poll on them.

1. Ext JS (http://www.sencha.com/products/extjs/)

2. qooXdoo (http://qooxdoo.org/)

3. Cappuccino (http://cappuccino.org/)

4. HTML5 and CSS3