The two mostly used WYSIWYG editors for drupal are TinyMCE and FCKeditor. I used this 2 great WYSIWYG Editor for my drupal 6 sites. At first I chose for TinyMCE because at that time because of two reasons. First it was most used on Drupal sites thus best documented and easiest to find support for.

TinyMCE

I use TinyMCE on 2 of my sites and it looks great. It has a lot of features. TinyMCE Installation:

INSTALLATION: Installing the TinyMCE module is a TWO STEP PROCESS. Users must download and copy the TinyMCE folder into the module directory as part of the installation process. This module only enables the TinyMCE editor to be used with Drupal. The TinyMCE editor itself is maintained by Moxiecode Systems AB and is used by many content management systems including WordPress. A browser compatibility chart is available at the TinyMCE website.

Setting things up went fast and easy. You only need to create a profile  then give an access permissions. Remember to choose some features you would like to add on Buttons and Plugins section. And in few minutes TinyMCE is integrated as your text editor.

TinyMCE offers a lot of features even I am not using at all. One thing is missing from TinyMCE, seems its not compromize with every themes. You can't configure the size if they are too large, the only thing you can do is removing some button or plugins and your TinyMCE editor will not have problems.

FCKEditor

I am using FCKEditor on this site and looks great. FCKEditor Installation:

INSTALLATION: downloaded and installed the Drupal FCKeditor module: http://drupal.org/project/fckeditor. Second, go to http://www.fckeditor.net and download the actual FCKeditor. Unzip the tar/zip in your ../modules/fckeditor directory. This will create a subdirectory: ../modules/fckeditor/fckeditor.

Installation went easy and to use it you dont have to configure lot of things (by default). It’s relatively lightweight than TinyMCE and by default I think it is enough for you as WYSIWYG Editor.

Conclude:

TinyMCE and FCKEditor both were great WYSIWYG Editor for drupal site. I use it on my sites. If you prefer to choose TinyMCE then you will find a lot of features include inside it but if you prefer to choose FCKEditor then you will find a lightweight and easy to setup editor.