There are thus 4 options for a WYSIWYG editor for MediaWiki:
By far the best is the VisualEditor extension. The main drawback of it is that it requires a third party “Parsoid” server and is quite difficult to setup. At CiviHosting, we take care of this for you however.
The advantage of the others is that they do not require a special server nor a special installation procedure. The disadvantage is that they are not as powerful nor as reliable as VisualEditor.