Pretty much all of ANH is gold to me. The end titles that go from the main theme to Leia's theme is incredible. They repeated it with ROTS even though Alderaan was in the film for about a minute!

The imperial march from ESB is probably in the top three themes of the saga (with the Main theme and force theme above it). Its as iconic as the indiana jones theme and is now in the pop culture.

Jedi has some nice themes. For some reason I really like Jabba's theme, which is this heavy bassoon thing that is buried in the mix but you can hear it on the cds. Williams lifted it from E.T. (when they are going trick or treating) but it works well for the tatooine overlord.

The prequel themes are a really mixed bag. Qui Gon got the best theme I think (all 1 minute of it heard during his first confrontation with Maul) with the romantic theme from Clones and the Anakin/Obi Wan duel also being highlights.