The best I've ever seen was a movie accurate Darth Vader costume - but more of an exact copy of his suit from the films. As a geek to be honest, that'd be awesome to have.
I chose going as a vampire for a few Halloweens in a row. All I needed were the plastic fangs that seemed to be all the rave when introduced, a white t/casual button down, jeans, sneakers and a cape which consisted of a black sheet of plastic.