Depends on your budget and the the licensing details, but I've always been a big fan of jQuery + ExtJS. jQuery is a fantastic tool for working with the DOM and a simple AJAX Framework, and throwing ExtJS on top of jQuery beings it all together. ExtJS is the best JavaScript UI kit that I've ever worked with. It's fast, never ran into a bug (but it has its quirks), and commercial support is available - if you need it.
ExtJS has a lot of features that a LOB application would benefit from, such as the desktop and charting. The charts are flash based but its part of the same package.
The fact it is commercial irks some people, but overall it's a well rounded framework that fills a lot of gaps.