Some requirements:
Flex Builder. And BONUS you can get it for FREE if you are a teacher or student.
I was interested in the comments on debugging above. I am one of the developers of the Amethyst Flex/AIR IDE for Visual Studio. We are very keen to have the best Flex debugging available so I would appreciate any feedback to help us achieve this.
The current beta of Amethyst has breakpoints, watch variables, locals, call-stack, step-into/step-out/step-over plus drilldown expansion in the debugging windows or in hovering debug tips in the code editor. There's a small movie showing a few debugging features of Amethyst here:
http://www.sapphiresteel.com/Debugging-Adobe-Flex-Applications
Shortly we will add more features (e.g. tracepoints and conditional breakpoints). But if there are any features we've missed, now would be a good time to tell us ;-)
best wishes
Huw Collingbourne
SapphireSteel Software
From the discussion above, below is the list of IDEs for Flex with debugging support