This really is not a question anyone can answer. You need to look at the framework and see what you want out of it. Some are bare bones, some have widgets, some do not require JavaScript coding. Most people who post here use JQuery.
Popularity should not be the measure you use. If that were the case, jQuery would win hands down.
You should evaluate the needs of your project and see which fits it best. That said, one of the reasons for jQuery's popularity is its versatility. If you have limited time for evaluation that's where I'd start.
In that case you should know a little about each of the major players. I'd concentrate on getting a good overview of jQuery, Dojo, and DWR. Prototype is a dinosaur, and GWT is very niche.
I don't have any particular company or specific job right now for which I can apply. I want to be prepared in advance. This is why I want to know which AJAX framework will give me more chances of getting job. I know other technologies like spring, hibernate.