I love Star Wars for what it is- a fun, simple, somewhat naive morality-tale, and the beginning of the Star Wars franchise. But to me, Empire is the best of the series. It's much more complex, darker and more twisted- which is right up my alley.

Darth Vader is considerably more menacing in Empire, which I also think is great. He sort of acted like Tarkin's thug in SW, but he really comes into his own in Empire- and he's one seriously scary dude.
I also love Empire's cliff-hanger ending, which was very daring and risky back in 1980. I love that they avoided the stereotypical Hollywood 'all-wrapped-up-in-a-neat-little-package' ending.

I also love Jedi- I thought is was a very exciting, somewhat bittersweet ending to the trilogy.