The best replacement for ckeditor is tinymce.

The best alternatives to ckeditor are rated as following:
★★★★★  tinymce
★★★  fckeditor
★★★  ckeditor
★★  redactor
★★  tinymce drupal
★★  kompozer
★★  aloha
★★  wysiwyg
★  tinymce 4
★  jce
  radeditor
  nicedit

Your alternative for ckeditor is not listed? Suggest!
Speaking about ckeditor, one probably shouldn't forget about ckfinder.

Other search terms related to ckeditor and their ratings are:
★★★★★ ckfinder
★★★★ image upload
★★ elfinder
★ file manager
You have in mind a companion for ckeditor and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of ckeditor in all years. Click on graph for more.
ckeditor
tinymce
fckeditor