I prefer TinyMCE over FCKEditor.It has better documentation & configuration options. You can set up & use in 1 minute.
[UPDATE]
However it seems to be not working out of the box when used with Ajax (ActiveScaffold).
Read this post for more updates.
[UPDATE 2]
The zipped size of FCKeditor v2.5.1 is 1.2MB and TinyMCE v3.0.zip is just 572.2 kB
