Just my another opinion. When I realized that Mootools has the smoothest animation, it became my favorite Javascript library. jQuery is very popular, especially for its ease of use. But in my book, smooth effects is ultimately more pleasing, while it is not much more complex than others too.
See the demos of both libraries on GUI effect called ‘Accordion’ below (and yes, try clicking on the ‘headers’ to see the panels’ slide effects):
Mootools: http://mootools.net/demos/?demo=Accordion
jQuery: http://jquery.com/files/demo/dl-done.html updated to http://docs.jquery.com/UI/Accordion (checked: 2011-07-31)