This is an offshoot of the "What is your favorite site?" post.  I saw the site for jquery and wanted to ask everyone who develops whats there favorite JavaScript Library.  Jquery, script.aculo.us or what?

jquery !

EDIT

ajax is this easy (and this is the most complicated it gets) in jquery

$(function() {

   $("a.myAjaxClass").click(function() {

      $.ajax({

         type:'GET',

         data:querystring,

         url:$(this).attr(href),

         complete:function(a,b) {

            if(b == 'success') {

               //do something with a.responseXML;

            } else {

               //error

            }

         }

      });

   });

});

prototype and scriptaculous

I use the ATLAS entensions is Microsoft Ajax toolkit verision 2.0 for .net framework 3.5.
It has it's quirks, but all and all it's a well developed and integrated codebase.
It has seamless integration in the Visual Studio IDE, which I LOVE