Imperavi Redactor is a nice & feature-rich WYSIWYG editor. I use Redactor for most of my projects, mostly through the Yii and Yii2 extensions. Redactor is a proprietary commercial copyrighted software but since Yii community bought OEM license you can use it for free with Yii.
Original developers of Redactor delivers some plugins for extending the core editor making it more convenient for using. At the same time it has a nice API for developing third party plugins to accommodate editor for many use cases.
Hope you will also find it useful, in case your development lies along the same line as mine.