The Empire Strikes Back
(20th Cent. Fox, 1980)
The second hardest thing to do in moviemaking, after making a good movie, is making a good sequel. Somewhat reviled at the time for being a blatant set-up for the finale, The Empire Strikes Back mostly pulled it off, advancing the story at the time in ways only hindsight would fully reveal. People tolerated it because it also expanded the view into the Lucas-verse by revealing the previously unseen Emperor, at the same time it brought in Yoda to flesh out the mystical Force a bit more, and deepened Leia's love interest without giving anything away. Totally unable to stand on its own, it's nonetheless an irreplaceable bridge in the series of films, and something you'd only miss if it suddenly wasn't there.