Well, it always depends on your project which library supplies the most useful and best functionality there is.
But my true love is and will always be Spry. Its build for designers, they made it feel like an extension of the web. And actually it has almost all features a designer and developer would need.. Accordions, tabbed panels.. And yes they also have "fancy" css like selectors which allow u to target elements using your CSS knowledge..
Just because libraries arent well known yet doesn’t mean they a worst than more known libraries..
I know it might sound like an advertisement talk comming for a Adobe Spry Community expert who is trying to lure to darkside.. But its not.. I would just suggest. Give the libraries a chance use it.. Don’t judge libraries by users blog posts thats only a users opinion.. USe it, experiment with it.. Only than u will find the library that matches your requirement.
which: Adobe Spry Framework for Ajax
Why: because it totally the shizzle