Ask HN: What is your favorite Javascript UI framework out there ?

I'm looking forward to build my next web app soon and this time, I thought I'd give a try to a fully featured JS UI library instead of reinventing the wheel. I haven't quite followed JS trends for a while, so I'd like to get some advice from this community. So far, I'm considering: - jQuery UI: has been there for a while, great community and tons of plugins but it's still very low level. - ExtJS: great UI but smaller community and the dual license scares me (I'm not sure I can afford the commercial license yet). - SproutCore: seems really great and love the UI, but writing all my back-end in JS kind of scares me. - Cappuccino: same as above. So, what JS UI framework do you like best ?