Well, it depends.
The oacs-5-10 branch has 3 supported editors
* CKeditor 4
* xinha
* Tinymce
In the head branch, there is basic support for CKeditor 5, which i did a few years ago in a weekend project.
CKeditor 4 has its problems, since versions after 4.22.1 require a license, and 4.22.1 has vulnerabilities (for most applications, these are not an issue, check yourself on Snyk).
CKeditor 5 is currently free of issues, but they have now a very "modular" design, where some of the modules (which were free with version 4) require a license as well. Furthermore, they advocate a composition with npm, which makes packaging for OpenACS more of an issue. I have heard, that Markus Moser picked up the version and added some more support, but I do not know the details.
xinha is still working, the project got some newer updates after years of a standstill, but still, maintenance is not great.
TinyMCE is regularly updated, and looks like a pretty solid option.
However, it is not so easy to give a general recommendation. If one needs just a basic rich-text widget, all of these might be useful. If one has very detailed requirements, custom plugins, support for mobile devices, etc. things might look different. ckeditor4 offers a version detailed configuration interface from the xowiki form fields (choose between standard, inplace, inline, templates, image selectors, allowedContent, various callbacks and skins), which are not available (to this extent and/or in a fully compatible way) for the other editors. If it comes to user-experience, end-users are sometimes very passionate about certain features. Moreover, on-site documentation with screenshots has to be adjusted, when the editor is switched, etc.
We still keep for OpenACS 5.10.1 CKEditor 4 the default choice for xowiki, but that might change in the future.
