What is the best website design software?

For learning, just a code-based editor without a design view with syntax highlighting (colours the coding). For that I'd recommend Notepad++.

If you don't want to learn, and just want to make quick pages, then Microsoft Expression Web Designer. It's probably the only time you'll hear me praise Microsoft software, and I don't think they're making profit on this product, but it really is the best I've seen. It produces clean, standards-compliant code (if you don't go into Options and add a background sound anyway). It gives out better code than Dreamweaver by far, and you can easily design using dividers as opposed to tables, it fits well with WAI level A, HTML, XHTML and CSS standards. Obviously it's not the perfect editor, if one could exist, but it's by far the best design-view editor I've seen.