There can never be an answer to any question like that. But, If you really stress it hard then you will get different answers by different people depending upon there fields and expertise.
The answer to your question by me would go like:
Web Development:
Adobe Dreamweaver, this answer will be given to you by most people because this tool is really good.
It allows you to code and test your webpage simultaneously and also provides an interactive environment for web development. In it's package you will surely find anything you ask for!!!!
Software Development:
Like, I said there can't be any perfect answer to this one!!!
It will depend upon the language you chose to write your code.
For me, I am a C# and C++ developer and "Visual Studio" by Microsoft is the thing for me...
It provides a perfect IDE consisting of all the development and testing tools you need...It supports other languages as well and you may go for anyone you prefer. (Since you didn't specify what kind developer you really are!!!!)
Again, the answer for "Web development" is perfect without any doubt but the answer for "Software Development " may differ!!!!!