I am an entry level C++ game developer, and I decided I want to switch over to Flash. C++ is awesome, but I love the fact that Flash Games are so easy to share, and I really want to try my hand on a new platform.
Which SDK is the best one to start learning the language. I am completely new to Flash so I am only looking to do some Hello World, and other ridiculously simple programs. But I still want the best development environment that would I could eventually use to develop games. If I have to pay for it, that is not a problem.
Assuming you mean which IDE not which SDK ... Use FlashDevelop. It's free and works well for people who are approaching Flash from a computer programming background rather than a graphic design background.
You would use Flex in order to compile for free, and there is an option to automatically download/install it when you install the Flash Develop IDE. You can also compile with the flash authoring software, but I don't recommend that IDE for programming as it's horrible in comparison.
But as the previous poster mentioned, sounds like you are talking about an IDE.
There are a few out there, the free one - Flash Develop is actually a really good IDE and I would definitely recommend it.
Adobe has Flash Builder and there is another one called FDT and neither are free.
There are a few others out there, and I would recommend googling "Actionscript IDE" or something like that. I know there are quite a few, but the ones I suggested are quite popular.