The frameworks I have tried out so far are:
I like them all; so far I find the event API (which has been my focus lately) of jQuery and Moo Tools to be the nicest of the five.
All hide browser differences from the user, something that used to make simple tasks take hours and hours to implement properly.
All of the libraries offer “minified” and compressed versions of their libraries, of the list above I believe Moo Tools comes in at the smallest when uncompressed when downloading their full package.
— Max Schubert