Since I had the same problem, I'm sharing the results of my research here:
ng2-ckeditor
demo here
ngx-editor
demo here
quill.js for angular
demo here
Note: Quill uses JSON to store the data internally, not html.
PRIME NG
Theme collection, that features a Quill-based WYSIWYG-editor as well, demo here. Some of themes are free.
ngx-md
A WYSIWYG editor which uses MarkDown rather than HTML, demo here
Edit
If your choice was quill.js, you might want to use KillerCodeMonkey's implementation since it is compatible with angular 5.
