Guys,
Give me a best advice .my company wants a drag and drop functionality in web browser , its an intranet application.

the requirement is below
There will be three windows within a dialog the user can drag one item in the tree from one box to another the box, should be resized when required
if a item in tree is clicked, its corresponding item in other box has to get highlighted And when mouse over happend there should be some info displayed.No glossy feeling, nothing

prototypejs.org
script.aculo.us

I use prototype quite a lot, although that's just because I'm used to it. I quite like it and find it very easy to use. Apparently mootools is just as good, but I haven't really had an urge to try.

GWT can't do drag and drop by default, you have to use the GWT Drag 'n' Drop extension: http://code.google.com/p/gwt-dnd/ I've had limited experience with GWT so I can't comment on it in detail, but when I did use it, I felt it didn't give the same freedom as something like Prototype gives.