Which WYSIWYG editor is best for a content management site?

StackOverflow uses Markdown syntax and the WMD Editor

Markdown is a very nice way to create valid HTML but its feature list is much shorter than the TinyMCE Editor. It might be quite useful for your users, provided they don't need the extra functionality.

Other options you may want to consider are:

• The What you mean is what you get editor.
• The YUI editor from Yahoo

I have enjoyed using WYMeditor in the sites we build using ExpressionEngine (a CMS).

Pros:

• will not allow invalid xhtml
• lets you specify a stylesheet for it list rules from so the user can easily apply styles to content
• code view for those who know html/css and need to tweak something
• supports image placement (not sure you can upload through it though, the CMS we use has its own file upload utility)
• it has passed the mom/client test for us without fail

Cons:

• will not allow invalid xhtml... sometimes you really really REALLY want to just give up and use that <center> tag

I use the Telerik RadEditor in my CMS. It's VERY robust, and has several hook-in points that allow you to extend its functionality to meet your needs. It doesn't have (afaik) the live view like the one StackOverflow uses, but that could probably be added if you so desired.

It does however provide the photo uploading/arranging functionality you mention, in addition to allowing you to upload documents and some other media. The downside is that the control is quite expensive, close to $1,000 I think. Fortunately, my company covers the cost :)