If you spend any time writing AppleScripts, you’re going to want Script Debugger. Script Debugger is the development tools the real power AppleScript users employ and it’s now up to version 7. It has a pile of tools not available in the native Script editor including a much better dictionary, better debugging, code completion, split pane editing, customization and more. 

I know AppleScript is a fairly niche thing these days but if you want to get better at it, this is the tool for you.