Lets skip the big comparisons here and get to brass tacks. Here’s the rapid review of Ajax frameworks:

• YUI -Stable and comprehensive but a little verbose.
• jQuery – Clean, fresh and perky but plenty of hacked, cruddy plugins to be wary of.
• Ext – Beautiful to look at but agonising to use.
• Prototype/Scriptaculous – Only popular because its used by Rails and CakePHP, but you can do better with jQuery.
• Dojo – Meh.
• MooTools – Trying to be like Prototype, little realising its picked thewrong one to copy.