I disagree with it being the most fun, because I would also consider TP/Delphi and C++ Builder.

Now I do agree that even something like Unity or PlayCanvas fail short of Flash's game development experience, because the browser debugging for 2D and 3D development sucks.

I have heard lovely things about Delphi’s and C++ builder’s fun factor (maybe from you on this very forum actually :) ), but I have not used those so I cannot speak to it.

At least of the stuff I have used, Flash is the most fun. Admittedly I never really got into any other kind of graphics-first development like VisualBasic and the like. Flash just spoke to me for some reason.