The theme when Yoda is giving Luke the "You must feel the force around you" speech. Chills go up my back to this day...
Also, the throne room at end of ANH is awesome.
Of course, asking me this is like asking me my favorite ice cream flavor...they're ALL good.
I agree with what someone posted above about The Duel of the Fates. Ep I blew cheese, but the music during the saber duel was awesome.
Music during the Obi-Wan/Annakin duel in Ep III was great too.