I've used prototype

Or rather evaluated (played with) it, as well as GWT, Yahoo UI, Dojo and a few others.
Far and away the best I've tried is Ext-js here User Posted Link
Best docs, cleanest code, etc.
A caveat is that a commercial license will cost a few quid.

If you have a Java backend, then I'd recommend DWR here User Posted Link for your remote calls as well. It takes care of all the marshalling and un-marshalling between javascript and java.

Posted By: yarmyyarmy, Oct 15, 10:09:59