Personally I'm also using Notepad++. Some will prefer Sublime Text 2.
If you have FlashDevelop installed for one of purposes, it provides similar functionality, providing syntax highlighting and word completion.
Otherwise, Visual Studio is pretty much a king among the free tools in terms of amount of intellisense it grants.
WebStorm is also good enough, but costs money (possibly making it less interesting target if you're after making a single plugin for now).