I waffle between YUI and Extjs. Ext is slightly smaller for the full stack of widgets and all, but in the way I end up using each, YUI is slightly smaller. YUI is better documented and somewhat better supported do to higher user count, but Ext has some nice widgets. Design of YUI seems a bit more stable, as well. Ext changes pretty frequently.
I've been using YUI for the past few months and it's impressive. Not quite as out-of-the-box as you'd hope but the capabilities are endless.
Also, I think it's time we stop mentioning Scriptaculous/Prototype in the same sentence as YUI, jQuery and Dojo.