I think the Flex Bulider and QTP plug-in are the most important. Since the Flex Builder and QTP plug-in communicate the Flex application with testing tool
It all depends on your budget, QTP is *VERY* expensive,
RIATest is not expensive option, but it is one man effort, although he is doing all the best in supporting the product via forums
FlexMonkey is complete free http://www.gorillalogic.com/stuff.flexmonkey.html, although I've never tried it, but looks promising.
We've been using FlexMonkey and it is quite good. It can be frustrating with very dynamic apps (lots of new components
added and removed at runtime), but I imagine any GUI testing framework will be.