I saw The Empire Strikes Back at the cinema,
I don't know how much sense this makes because I would only have been 3 when it came out, so whether they were still showing it two years later I don't know, but it was definitely the one where it's revealed that Darth Vader is Luke's father, because I remember asking my Dad afterwards why he isn't called Luke Vader.
( Moon Girl Technologies horrendous beanbag, Thu 7 May 2009, 22:49, archived)