The built-in editor. With LispWorks I don't use GNU Emacs. I have GNU Emacs configured, so that I could use it, but I don't... I use SLIME with SBCL, but mostly because of SBCL, which is a great implementation.
I'm faster and more in the flow with the LispWorks IDE.
Especially the REPL of SLIME / GNU Emacs.
LispWorks provides a better REPL experience.