The tragically underused "Luke and Leia" in Return of the Jedi, which may very well be the best music in the entire saga.
"The Battle of Hoth" (part 1, part 2) is almost 15 minutes of sheer awesome. Highlights include the anthem for Humongous Mecha (part 1, 4:02-7:40), some preposterously heroic music when Luke pwns the AT-AT with the grenade (part 2, 2:45-3:40), and what can only be described as a gleefully malicious version of the Imperial March (3:40-4:10). Good times.
Say what you want about the prequel trilogy, Episodes II and III have brilliant ending themes. Episode II has the first prolific use of the Imperial March as a major foreshadowing, dovetailing into an awesome rendition of "Across the Stars". Episode III has three minutes of no dialogue which lets John Williams flex his arms like he wants to, cycling from Padme's funeral theme to Leia's theme, then to a small snippet of Harry Potter-esque music before launching into the Force Theme over a binary sunset to create one of the most awesome bookends ever.
No mention of the ending theme is complete without the extended end credits on the soundtrack, which includes an amazingly bittersweet rendition of the Throne Room theme from A New Hope that perfectly captures the mix of despair and hope that the end of the movie conveyed.
The Revenge of the Sith soundtrack was basically one whole emotionally charged Crowning Score of Awesome. The opening credits where Anakin and Obi-Wan are flying together as friends and triumphant heroes, Grevious's choral theme, the eerie, guttural music playing during the Mon Calamari ballet as Palpatine begins to ensnare Anakin with whispers of the power of the Dark Side...all wonderful.
"That's no moon..." Especially brilliant since it's the Darkest Hour of A New Hope, and Williams could have scored it with something soul-crushingly hopeless. Instead, we hear an incredibly rousing version of the Rebel Fanfare to remind us that our heroes aren't licked yet...
The main theme threaded throughout that cue isn't the Rebel Fanfare, it's the Imperial Motif. (Not the later and more famous "Darth Vader's Theme" Imperial March, which Williams didn't compose until Empire.)
During the Death Star battle score for A New Hope, it's noticeable that the music changes from scary/foreboding to confident/heroic at the exact moment Luke turns off his targeting computer and trusts the Force. That's the turning point of the movie and the music (literally) underscores it.
In one early interview, George Lucas said he had very high expectations when he made Star Wars in the mid-1970s. The only thing that exceeded his expectations was the music.
Jedi has all the best themes of the original trilogy, such as "Leia's News/Light of the Force". Starts off with Luke and Leia's theme, rolls into the love theme, and then the EPIC force theme at the cremation. But pretty much the whole of the second half of Jedi has awesome music.
Afterwards "Victory Celebration" is just...amazing. People often call it the only good change made in the Special Editions.
Not one of the flashiest themes, but the Jawa Theme was otherworldly.
"Destruction of Alderaan".
"The Tractor Beam/Chasm Crossfire", but especially the second part. Starts out with some cool spooky music, then eventually translates into an amazingly uplifting version of the main fanfare, intermingled with Leia's theme and some other great stuff. Combined with the bottomless pit, aka the "I think we took a wrong turn" scene, it's just pure awesome.
"Padmé's Ruminations" where Padmé looks across Coruscant at the Jedi Temple from her apartment and Anakin in the Temple seems to look back. Featuring the only use of the One-Woman Wail in Star Wars music.
"The Rebel Fleet/End Titles". For a film that ends on quite a downer, the beginning of this piece focuses purely on the hope for the future with a stirring rendition of the "Han Solo and the Princess" theme which ascends into the standard End Titles music. Cutting this theme a bit shorter than the other films, it segues into reprises of "Yoda's Theme" and "The Imperial March" that are anything but unnecessary repetition. After that, the most soaring rendition of "Han Solo and the Princess" builds and builds until we are presented with what is possibly the greatest climax to a score in film history.
"The Throne Room" ending theme from A New Hope. You will rarely find another closing score that embodies pure triumph and happiness (and of course the cute fluttery riff from the main theme when we find out R2's okay after all). Another theme plucked in it's entirety by Family Guy (before their own parody trilogy in fact).
The main theme from Jaws. Spielberg reportedly assumed the unusually simple theme was a joke when Williams first presented it to him. But just try to forget it.
The Flying Theme) from E.T. The Extra-Terrestrial. It's particularly neat in how it times up with the bicycle flying "against" the moon; it's like you can hear the wheels turning. Spielberg stated himself that technical wizardry and special effects can get those bicycles to fly, but only upon John Williams' strings do the characters become truly airborne.
Christian Bale (at a very young age) singing "Suo Gan" (a Welsh lullaby) in Empire of the Sun, just as the Japanese Kamikaze pilots are about to take off for their mission. You'll cry at about the same moment the Japanese characters in the film start crying because of the sheer gallantry of it all.
The sheer joy in this piece, "Exsultate Justi", as it plays in the end when Jim makes it back to the now-abandoned internment camp with virtually no hope in sight, but in spite (or because) of this, he still rides his bike around the inside of the buildings like the little kid that he is.
Very effectively used in the helicopter scene that begins with a growing sense of urgency as Lois faces greater and greater peril until Clark Kent appears. When he realizes he has to change, the growing musical swell that climaxes when he rips open his shirt to reveal his chest symbol is a classic of the superhero film genre.
A leading example is when Lois Lane's family is trapped in Luthor's sinking ship. The music begins with a mournful dirge as the trio are pulled under to their seemingly-inescapable doom. Just when the tragedy becomes too much, two red boots appear on the door window and the March blasts out with full thunder as Superman saves the day!
Smallville ends with Clark Kent running out onto the Daily Planet's roof, ripping open his shirt to reveal the S-shield beneath as he prepares to take flight, the camera focusing on that iconic symbol all while that classic Superman theme song plays ever so magnificently in the background and into the end credits.
Oh hey, since we're on the topic of Superman, may we present his love theme.
In addition to the main fanfare and "Can You Read My Mind?", there is a really moving musical moment after young Clark Kent tells his mother Martha that he's leaving Smallville to pursue his destiny up north. It's called "Leaving Home", and starts a minute-and-a-half into that track.
Much less well known, but just as awesome as all of the above, is the nine-minute piece from The Towering Inferno that accompanies Paul Newman and Steve McQueen rigging explosive charges to the burning skyscraper's water tanks. One of the greatest examples of music used for a rising tension/countdown effect.
We all know John Williams can compose music for virtually everything, but did you know that he also scored a Western film? (Even better, a John Wayne film?) Check out this rousing piece from ''The Cowboys''.