+1 for jQ

If it's UI-based, I'd use ExtJS. Otherwise, MooTools. Both of these are very performant and give you a lot to work with in a neat package. jQuery UI, MochaUI, etc. are all miles behind ExtJS when it comes to behaving (reliably and efficiently) as a real tookit (with widgets and layouts).

In my opinion, jQuery is suitable for progressive enhancement but not for dedicated apps.