CKEditor 4 excels in two things:
-Handling selection - especially tables.
-Editing for accessibility.
I would recommend v5 though, otherwise you'd be potentially giving my one good friend, who's already very busy maintaining v4, even more work. :D
https://textbox.io/ - these guys gave us a run for our money a few years ago with their paste-from-external-source (e.g. Word) capabilities, but I believe they've been since acquired by Tiny.
https://github.com/tinymce/tinymce - I think this one is slowly being cannibalized by TextBox. I remember them receiving a lot of funding at one point, but getting nowhere with it.
Froala - avoid. It's not really open-source, and judging how support for paying customers works you're not getting your money's worth.
Quill - we've never seen them as a threat, so maybe they're not that good? Especially given that Slack thing that's currently unfolding.
Edit: Formatting.