best WYSIWYG editor from the UX side? There are a bunch of WYSIWYG edirtors available. I've tried CKEditor so far (and tinyMCE with wordpress). CKEditor is a very fancy one but fairly complex and weights 6Mb including source files.

What is in your experience the most buggless and stable? 

It needs to have a support for IMCE bridge module to be able to get images from disk.
I personally use tinyMCE on my sites, it's not heavy, and you can use the IMCEbridge.

This module will install the WYSIWYG API, then follow these instructions for adding TinyMCE or any other you want.

The IMCE Bridge module can be found here and is compatible with CKEditor, FCKeditor and TinyMCE.