Personally I think that the Darth Vader theme by John Williams is one of the best character themes. What is amazing is that everyone can hum it, yet it is actually quite chromatic. It has a simple rhythm, and so sounds simple. Somehow John Williams turned a chromatic scale into such a hummable melody to go with the rhythm.
Another great John Williams theme would be Across the Stars from episode 2 of the prequels. No matter what you think of the movie, that is one great melody.
By the way, William, I don't know if you are aware of this or not, but the march you refer to in the end of the first Star Wars (commonly called the 'Force Theme' among John Williams fans) is actually probably the most used theme in all of Star Wars. You might be surprised to find out how much it underscores. Even in the prequels, which do not reprise all the themes at all, it is there very strongly. On every episode it has a very strong position.
Now, so that I don't appear to be too much of a John Williams fan (which I am not) or a Star Wars fan (which I am not) or a Star Wars music fan (which I might be), I will give my non-Star Wars theme: Gods and Generals theme by John Frizzell. It is really not complicated orchestrally or compositionally (I don't think), yet it perfectly sums up the feeling of the movie (or perhaps how the movie should have been). Even if you didn't like Gods and Generals, it is very worth getting the soundtrack, if only for that track.
Colin Thomson