If I absolutely had to pick one, probably Imperial March. I'm partial to marches. I agree with the above that the whole piece is awesome and a bit of a shame that more of it isn't incorporated into the movies.
Other favorites are the Force theme, the Asteroid Field, and the Return of the Jedi. Into the Trap mentioned earlier is a great one too, I think it might be the most "narrative" of the scores, practically telling you what's going on as it unfolds.