Ive used cairngorm, parsley and mate, I would choose Parsley - If you are comming from java background I would go for parsley. It is not a JQuery kind of framework - it is a design framework, like best practices to be followed. Flex itself is a framework for actionscript programming.
There are several Flex and Actionscript architecture (MVC) frameworks. I currently use Swiz. Below are some links to architecture frameworks for Flex and or Actionscript: