What's the best webapp UI you've seen and what was used to make it?
dpcan: I don't think it matters what it was made in.  You can use ANY language, tool, JS, whatever, as long as you are a talented UI designer.  I think people tend to think that they can make these great UI's faster and easier by using certain tools, but I don't believe that is the case.  YUI may make it faster to create Javascript tables that can be sorted and searched on the client-side, but you still have to learn that system and integrate it into what you have already created.  Pick a language, a JS framework, etc, and master it and any UI will be easier and faster to create.