What is the best javascript library available for implementing ajax and user interface components?

Well, if you're committed to using Scriptaculous, the "library" you download already comes with the foundation for it called Prototype.

Scriptaculous implements the base code found in the Prototype, as well as the effects like the slider, drag/drop, etc.

Another library that I've been spending some "quality" time with lately is the Dojo library. It also has drag/drop and a few of the effects you would expect to find. I will say it's a bit heavier than the Prototype/Scriptaculous package, but not by much ~50k if you only add the .js files to your project that you actually need.

I think the best documented is probably Prototype, but there's great peer support out there for both.

At this point, I'm probably going to go with Dojo as my "standard", and then implement others as required, such as Rico for their grid, etc. Scriptaculous is a great package, but the apps I'm working on require too many things outside of Scriptaculous (layout tools, primarily) to make it my "standard".

Check out the Dojo project at: http://dojotoolkit.org/
and it sounds like you've already got the Scriptaculous/Prototype package.

I hope this helps.

I think your choice should not go out from jQuery or YUI.

that s right and jquery realy the best
i built a cms with jquery that s work fine i publish that in youtube
http://www.youtube.com/watch?v=ug4uvAK2O...