The Godfather, Part III
Um, without a doubt, The Empire Strikes Back.
It's dark. It's epic. It's funny. It's romantic (even though it has, what is in hindsight, a really icky kiss). It has an absolutely amazing score that equals and, in some ways, surpasses the original's iconic themes. It contains one of THE best lines in movie history ("I love you." "I know."). And it has one of the most jaw-dropping plot developments ever (I still remember yelling out "NO! NO! NO!" in the theater when I was all of 4 years old at that particular point in the film).
It's highly likely that Empire played an important role in ultimately shaping my taste in movies. It had a downer ending where the heroes didn't win; I have a particular affinity for early-to-mid-70s American cinema with its bleak outlook on just about everything. It pulled the rug out from under its audience; I love movies with brilliantly conceived and executed plot twists that cause the viewer to re-evaluate everything that has come before. It had snappy, romantic/comedic dialogue between Han and Leia; one of my favorite film genres is the screwball comedy (particularly The Lady Eve, Bringing Up Baby, His Girl Friday, and Peter Bogdanovich's homage What's Up Doc?).