Actually, if you think it through, 'Revenge of the Sith' is the most likely...
After all, Epsidoe VI was meant to be 'Revenge of the Jedi', until it dawned on them that revenge was, as they had written it since the beginning of the whole storyline, the Dark Side, and so not a Jedi 'thing'. Hence change to 'Return of the Jedi'.
As revenge is Dark Side, and it is the Sith (well, the Emperor) who is carrying out what is nothing more than a grab for power aided by a revenge motive, then it is a VERY fair idea.
But it could as easily be 'Return of the Sith', just to tie in the halves of the stories from the offset, Episodes I through III are
Act One -
Return of the Sith
and then Episodes IV through VI are
Act Two -
Return of the Jedi
Does have a nice poetic ring to it...