It actually depends. This is what MLA refers to as an "edition" (see section 5.6.12 of the MLA Handbook if you have it) -- a classic work that gets published later on, with the editor selecting the version of the original work to include, adding explanatory notes and an introduction, etc. Regarding these, the Handbook states:
"To cite an edition, begin with the author if you refer primarily to the text itself; give the editor's name, preceded by the abbreviation "Ed." after the title."
Then it goes on to say later:
"If your citations are generally the work of the editor (e.g., the introduction, the notes, or editorial decisions regarding the text), begin the entry with the editor's name, followed by a comma and the abbrevation "ed." and give the author's name, preceded by the word "By" after the title."
So essentially it depends on what you are "primarily" citing in the body of your paper -- the original work or the editor's notes.