I have used Spry and YUI.

YUI is quite good. Support has been prompt and new updates are regularly made.

Also YUI will host the .js files which is nice of them.

It might have the most built-in features at this point but I haven't used others enough to say that with confidence.

It is a big library though. Oh also lots of cross platform browser behavior built in. So in my opinion, YUI is my favorite.