As always, the term "best" is vague at best. There are a number of Ajax frameworks out there and there will certainly be more coming in the near future. As the Ajax space matures I'm sure there will be consolidation in the framework arena where the best will continue to survive and the others will go away.
In short, do a Web search for "Ajax frameworks" and you'll surely get plenty of hits. Take a look at them -- each one does things a little differently and some may fit your needs better than others. Choose wisely -- remember, some of these frameworks may not be around in a year or two.
Ryan Asleson<br />Co-Author, <a href="http://www.amazon.com/Foundations-Ajax-Foundation-Ryan-Asleson/dp/1590595823/sr=1-1/qid=1158926537/ref=pd_bbs_1/002-7412154-2924860?ie=UTF8&s=books" target="_blank" rel="nofollow">Foundations of Ajax</a> and <a href="http://www.amazon.com/Pro-Ajax-Java-Frameworks/dp/1590596773/ref=sr_11_1/002-7412154-2924860?ie=UTF8" target="_blank" rel="nofollow">Pro Ajax and Java Frameworks</a><br /><a href="http://taconite.sf.net" target="_blank" rel="nofollow">Taconite</a> Lead Developer