I've used TinyMCE and I must say I'm not a fan. The main issue with it is that when new versions are released, the API can change drastically, making it very difficult to manage upgrades. There seems to be little regard for backwards compatibility.
The YUI editor is quite nice, and really easy to use. It's still beta as far as I know, so I'm not sure I would necessarily recommend it in a production environment.
FCK is a nice editor, and it has ASP.NET support.
