I'm sure you could argue it either way. I think that at the beginning of the movie, we are lead to believe that it is Luke - it's like, "Look, Luke's back, and now he's a Jedi Knight!".

But Yoda and Obi-wan throw us off-track after that - they tell Luke that he's not a Jedi until he confronts and defeats Vader. HOWEVER, I think we can all agree that Yoda and Obi-wan were mistaken about Vader - there was still good in him - and Luke was right.

So by the end of the movie, I believe that the revelation we are supposed to have is that the 'Jedi' that returned was Anakin. He came back and put things right.

But having said all this, I believe the title was chosen so that it could be interpreted all three ways: the 'Jedi' as Luke, Anakin, and the Jedi Order as a whole. My rewrite of Episode III is going to be called 'Fall of the Jedi', which operates on the same notion.