Anyone who doesn't say Empire Strikes Back clearly needs to be frozen in carbonite.

I think A New Hope and Return of the Jedi are both pretty close in terms of quality, but it'll always be Empire Strikes Back that wins it for me. I love the very real struggle of the Rebellion against the superior Empire, I love the development of Han and Leia and there's just so many great bits. Oh and Darth Vader gets lots of screen time which is a definite plus as far as I'm concerned.