hard to say because some scores are beautiful but the best parts are not really recorded in the movies. The completed soundtracks are always better than the music recorded and edited in the movie. I had a 20-bit skywalker sound Star Wars Trilogy CD and lost it :(
overall, I still say the opening theme. nothing comes close in any movie I have seen. If the Star Wars opening theme is not the greatest score in cinema history, then haven't I watched a movie that has the better score yet.
ANH just has the best collection of scores and was put to use the best way: force theme (binary sunset), throne room medal ceremony. 'Here they come'. even the jawa theme! we here stories of people that were inspired to go into filmmaking because of Star Wars. I know a few people in high school that got into orchestra music because of star wars.
ESB: if you listen to the imperial march on the soundtrack its awesome as hell. but i always felt the movie could have utilized it better. you hear the theme over and over again, but its always the same first dozen or so notes. the SE made it worse. Yoda's theme is just moving. the little piece with luke encountering his projection of vader in the cave is excellent. as is the scene where frozen han is being escorted thru cloud city as luke watches on.
ROTJ: the music helps this movie a lot and has best collection of scores after ANH. Jabba's theme. LAPTI-NEK (flame away). the litte piece that starts as soon as yoda passes away is just beautiful. JW had to come up with something more menacing than the imperial march for palpatine and absolutely nails it! The Ewoks have their detractors but the ewoke theme and battle for endor theme is great. Last but not least - its not in the movie, but in the closing credits, the last minute when everything comes to a cresnedo. ROTJ may be the weakest of the OT, but certainly not the score.
TPM - duel of fates is probably the only piece of PT score thats as played as any of the OT scores. augies municipal band was great too. trade federation music was alright. The funeral music is a hard one because I associate it with ROTS more even though it originated here.
AOTC - the scores start to become less memorable :( the coruscant chase did not sound anything special. geonisis had no unique theme, neither did the droid factory. Some reports indicate this was due to lucas not having the film ready for scoring :( some of the incidental music, like when obi-wan is spying on dooku and when mace/obi-wan/yoda are talking at the jedi temple at the end of the movie, creates great mood. The only full blown theme was the across the stars love theme, and its great. I really liked the part that starts at 2:27 - but its not in the movie
http://www.youtube.com/watch?v=9nk_WHHTQtY
ROTS: the best scores of the PT. I like order 66, obi-wan versus anakin. My favorite is Padme's Rumniations but in the actual movie, you cant hear it very well - also feel there should not have been the voice over. I like grevious theme. But there is also music that I felt misfired. I really don't like the mustafar music. In fact, the entire sequence starting with Obi-wan arriving on tantive IV all the way thru obiwan and yoda discovering anakin killed the younglings just dind't work for me. came off as too operatic and i don't think it was the right tone. would have prefered the music more like pschyo or omen or shining to create a true sense of dread and horror. just like when obi-wan says "im so sorry" and we cut to a scene of anakin all alone in mustafar. that was pitch perfect.
still amazing how JW went above and beyond the call of duty for Star Wars and created a score that just rocketed beyond anyones wildest imagination. The above pieces would practically be a career for any other composer, but for JW it was all in the same saga!