I know this sounds like a cop-out, but the answer really depends on you and your environment. For example, Dojo and Prototype provide many similar features but they're often accessed or invoked differently, so you'll have to decide which style you prefer. DWR is another powerful tool, but while it does a great job of abstracting the connection plumbing and type conversions between JavaScript and
Java, it doesn't have particularly powerful support for updating the DOM, like Taconite does.
The bottom line is there is a lot to choose from. Search the web for some examples on each one you're considering and take a look at features, coding style, and other things that might influence your decision.
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