My favorite is Mootools, too.

That said, the downside to Mootools is that it does not play nice with the other kids at all. Running it at the same time as you run another framework is usually a recipe for disaster. However, I don't know of any features that other frameworks offer that Mootools doesn't have, so it usually hasn't mattered for me.

My endorsement isn't all that impressive, though. I'm still something of a Javascript newbie myself, and my main experience has been with Mootools and YUI, so I can't really intelligently compare Mootools with jquery, etc.