1. Browser WYSIWYGs are really hard to make.
2. All browser WYSIWYGs are terrible.
Avoid rolling your own WYSIWYG if you can. Even if you have a team of 1500 people at your disposal.
I worked on CKEditor 4 (very different from v5, which is a rewrite).
I've written my own native Windows WYSIWIG editor and a fair amount of web UI, and was thinking of rolling my own web-based editor.
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.
TinyMCE version 5, released earlier this year, incorporated much of the Textbox.io features and technology and is the main product moving forward. It is better than Textbox.io in almost every way now and is recommended for new projects.
Tiny did indeed raise $4M in venture capital last year. It has not been squandered by any means. In fact, we have only just started spending it as we were profitable and growing when we first raised the money.
Tiny has a good-sized development team with more than 30 people in engineering, QA, design and product management. Of the many options out there, TinyMCE is a good bet!
(disclosure: I am a founder and the CEO at Tiny)