What do you think which is best IDE for coding ActionScript-3 with most of the support..
I know following IDEs..If anyone knows any other better...
For "with most of the support", if you mean the official Adobe one, it must be FlexBuilder(FlashBuilder) or Flash IDE. If you mean the community, I think all of them are very well supported.
And when you mean "coding", yes, Flash IDE is horrible. And actually you do not need it for debug since both FlexBuilder and FlashDevelop(with some tricks) are able to debug.
Finally there are at least two more IDEs, axdt and FDT, both are eclipse plug-ins.
Practically speaking there are really only three contenders: FDT, Flash Builder and FlashDevelop. Of these two, only FDT and Flash Builder are cross-platform, Flash Develop being Windows only, with no foreseeable port to OS X or Linux. Sepy is OK, but was last updated in 2005.
If you're Windows-based, and are only interested in an IDE for Actionscript, then FlashDevelop is an excellent choice and, being free, the best value for money.
The other two - Flash Builder and FDT - have the advantage of being Eclipse based. This means that you can install a huge range of other plugins to make it a comprehensive web dev environment. I use FDT with Aptana and the combination is awesome: in one package I have the ability to develop Actionscript (with support for Ant tasks, debugging with MXMLC), PHP, HTML, Javascript, XML and SQL queries, with code hinting for all the major JS libraries like JQuery and Ext, PHP libraries like Zend and Doctrine, etc, etc.
Between Flash Builder and FDT, I use both regularly and have to say that for me FDT is the clear winner: support for live error highlighting, refactoring, code completion and formatting is simply much better.
Addendum: As mentioned elsewhere, I should have flagged TextMate as another solid - if less comprehensive - IDE: it has support via bundles for AS3 and MXML and ties in nicely with the Sprouts development framework for testing and debugging. Mac only though... albeit I hear good things about the E-TextEditor clone for Windows.
Some good answers here. I'm not usually an IDE kinda guy but I've used both Flex Builder (soon to be Flash Builder) and FDT on projects and I have to admit they can come in handy. Having a debugger can occasionally make development and debugging much easier. Plus Flex Builder 3 has a built in profiler which is pretty useful. I haven't used FDT recently (when I did it didn't have a debugger) but Flex Builder 3 isn't all that "heavy on system" (both are based on Eclipse).
I might add that using a plain-old text editor (e.g. TextMate) can be a good experience. You'll have to learn to deal directly with the compiler or use ant to manage your builds which helps you learn about an important part of the tool-chain. This isn't exactly what you asked for though - so unless you are interested in learning these things an IDE is the faster, simpler option.
In my day-to-day use, if I am writing a 1-2 class project (like an example I will post to stackoverflow) then I use TextMate and build from the command-line using mxmlc. Pretty much everything else is in Flex Builder now (probably to justify the $$ it cost me).