The best way to learn HTML is to actually write the HTML code in a text editor. That being said, most WYSIWYG editors allow you to edit the code they generate, so you can see how it's built.
The problem is that many editors generate clumsy, non-optimized, and non-compliant HTML. As long as you understand that fact and take the code with a grain of salt, then here's my recommendation:
Macromedia Dreamweaver
Dreamweaver is now owned by Adobe. It is a very robust editor, containing one of the best text editors as well, so once you graduate from WYSIWYG to editing code directly, you can still keep using it. Plus, the HTML it generates is actually not too bad compared to most other editors.
