What are the best WYSIWYG editors for Ubuntu OS?
For Free: Kompozer
For Pay: Quick 'n Easy Web Builder. You can now find this in the Ubuntu Software Center and it is a paid application. It has some really unique features that allow you to design by just placing images, text, videos etc on a page and it produces the html5 code and css for you. It even has basic image editing capabilities inside the application. It is not an html editor however, although you can tweak the code to some extent inside the application. It saves files in its own application format and only produces html code when you publish your page or site. So you are free to edit the code/css once its published with an html editor, but doing so no longer allows you to edit within QnEWB. You have to go back to your original source file to make any changes and publish again.
Another restriction is that since you are placing images and text wherever you want on the page, it uses absolute placement, so you can not produce pages that automatically re-size to a browser window. You would need to edit the code after its published to make those changes.
With those minor limitations in mind, it is a great WYSIWYG editor and I highly recommend the purchase. (ie I am not affiliated with QnEWB in any way or form.)
Kompozer is also good -
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I assume you mean WYSIWYG HTML editors -- I was going to recommend NVU, but it looks like BlueGriffon is the successor to NVU.
Give it a try: http://bluegriffon.org/