In my experience you need to know more about what you want to do before you can really pick one.
There are quite a few frameworks that make this easier, but it can still be done without them.
Flex is a great technology and makes MVC really easy to implement, so as long as you observe good coding practices you don't need anotehr framework
(I've never been a fan of Frameworks for frameworks)
The word that you're looking for is "integration".
Flex supports the MVC pattern right out of the box if you make use of the code-behind approach in which MXML files are strictly your view, the root element of the MXML is a custom class which acts as your controller, and of course supporting model classes are used in the controller. It works pretty well as long as your discipline yourself.
There are at least several frameworks available that take the MVC pattern organization to the next level. I've read a bit about them, but don't have any experience to offer. If you have a project with significant enough complexity, then you will probably be able to justify the overhead (learning curve, setup, and runtime resources) of using one of them.