My personal favorite is "Into the Trap" from ROTJ. I love this build-up. Williams stacks more and more on top of the same piece of notes. Another one is the Asteroid Field from ESB which is so wonderfully dramatic and soft at the same time. Third one would be the Emperor's March. I have heard that one so often that I couldn't stand it for some time.

The funny thing is the more I think about the more excellent themes there are. Williams really did a fantastic job in making it so easy to listen to that you sometimes forget that it's a score. While some scores just don't work without the movie attached to it, Williams' scores work without any visual material. Partly this is why they are so popular. I guess.