The question is what you want to do and what your skill level is.
You can develop your skins with Notepad alone....
Even if you don't need WYSIWYG this might bee a bit basic.
I would use a editor with a HTML checker.
You could use
NVU (open source too)
I like "
Max HTML Beaty" better (I don't think it is being developed anymore, but I really like the configuration options for HTML Tidy)
A lot of people use Homesite and Ultraedit too.
If you do want WYSIWYG
(this still generates more dirty HTML)
Dreamweaver is the best there is.
Some people use Photoshop (and the Slice tool) but I almost never use it that way (although I'm a Photoshop teacher).
For debugging your skins / HTML there is only one tool I think.
The Firebug extension for Firefox.
But the best tool is still....
The best book on CSS you can get.