It all depends on your needs really. I think jquery is most popular toolkit, but YUI or dojo - these ship the infrastructure to build all kinds of widgets, + wi

It all depends on your needs really. I think jquery is most popular toolkit, but YUI or dojo - these ship the infrastructure to build all kinds of widgets, + widgets themselves - which is NOT same jqueryui project. You get a lot more in terms of widgets with dojo and YUI - also they are maintained and tested by their teams, so you dont have to worry about compatibility with future releases - like with 3rd party jquery ones. Personally I suggest you give a try to dojo - it's equally fast as jquery if that is important, I used all 3 of them, and evey framework has it's strengths and weaknesses. I'm most happy with dojo, and YUI comes 2nd. Also if you are Zend Framework or Django user, there is Zend_Dojo and Dojango that allow you to integrate smoothly if you are into this kind of solutions.