The duels in The Good, the Bad & the Ugly, Once Upon a Time in the West and For a Few Dollars More, all collaborations between the composer Morricone and the director Leone, I would humbly argue are of the highest achievements in the coordination of music and film.
The greatest cinematic duel for my money goes to that near the end of For a Few Dollars More. A total of 12 words are spoken in the entire scene, all by Clint Eastwood's character. The build up to the duel and its conclusion lasts just over 4 minutes, but Leone is capable of packing a bucket load of ideas into a single shot, while Morricone stretches the sound and music to its boundaries, so it seems much longer.
The three players are the western genre greats Clint Eastwood, Lee Van Cleef and the Italian, Gian Maria Volonte. All three are on form. The cool swagger of Eastwood; the confident, stern, unmovable Van Cleef; and Volonte, who here plays the psychotic, misanthropic, drug-addicted and murderous villain.
The duel begins with Volonte extending a musical pocket watch.The music we hear is the same as that heard by the protagonists. Volonte is here to kill Van Cleef in cold blood for crossing him and stealing his loot. Now started, we understand that when the jingle stops, guns will come out blazing. But Van Cleef is unarmed, having been separated from his rifle.
Morricone blends the off-screen soundtrack into the music heard by the characters, taking the simple sound of the pocket watch and building it into a lush canvas of sounds. At 20 seconds, we get Morricone's strings. They coincide with Volonte's moment of enlightenment. He looks into the watch which reveals a picture. It's the sister of Van Cleef's character. The watch he took from the dead body of the young girl he raped and murdered years ago is unwittingly revealed to Van Cleef. Morricone's slow and ominous strings punctuate the moment, and the build up is taken to a whole new level. Van Cleef is aligned in the background. No words are needed. 
Volonte is the star here. It is his face that Leone loves. Watch the slow movements, the head tilts from Volonte, the way Leone frames his movements. There are many subtle treasures of brilliant acting in these few seconds. We see the black, psychotic eyes of a murderer, but also a character who appears much more unsettled than earlier in the film. Van Cleef on the other hand is stoic. There's no movement except an ever-so-slight curl of the lip. He knows he'll either kill or be killed. He's accepted all possible outcomes. 
Then comes a heartbreakingly beautiful montage of 12 shots, starting with an image of Van Cleef's rifle, his face, then a close up of Volonte's hand, the hand that is about to reach for the gun that will execute Van Cleef. The scene is jolted as Morricone cues a second, identicle jingle and we're led to another hand elegantly outstretched, holding a second watch. And more hands. This time its Van Cleef's reaching into his vest to find his watch gone, now held by Eastwood. The dual jingles represent the dueling sides, the fate that has brought them face to face. Their destinies have finally, rightfully synched in time in a fight to the death.Eastwood is given a splendid entrance as the camera pans from the watch in his hand to reveal its holder. It's one of the archetypical images of Eastwood's western period. If you were to freeze that frame, it'd be instantly recognizable for most people. They may not know who it is, or from which movie, but it is an indelible image in the public imagination. The signature cigar shift is classic Eastwood. Some cineasts have interpreted this Man With No Name character as a guardian angel. Indeed, he saves Van Cleef from certain death at the last minute.
Morriconne brings up the strings and a guitar shatters the scene at 1:23. To me, this rather than the duel's outcome, is the real climax of the scene. It is a cinematic orgasm. A perfect union of sound and picture. Volonte gives an elegant spin of the head, then a piercing, cold stare, flashing that million dollar face. The music builds to bursting point and seems to be the force that whips Volonte's face in the direction of Van Cleef. It's a stunning portrait. He now realizes Eastwood and Van Cleef have been in cahoots. The look of rage and betrayal in Volonte's face. All the players are clued in on the game. Their eyes meet and they'll stay that way for the remainder of the scene. All the while Morricone is frantically plucking the guitar in that Spaghetti fashion.
An open door flutters in the background
Eastwood holds off Volonte at gunpoint and approaches Van Cleef. He hands him a holstered gun and announces, "Now we start." Morricone cues the mariachi-style trumpets and the players take their positions. Eastwood sits cool-ly in the middle, not quite a mediator as there can be doubt whose side he's on. Now that he's made it a fair fight, he's only a spectator. He sits not unlike Toshiro Mifune in Kurosawa's Yojimbo, undoubtedly a great influence on Leone.
At 3:31 Morricone's soundtrack is given a death of its own in the form of a haunting hollowed bass drum. We can almost imagine the music has been a kind of a vicious sandstorm that whipped through and turned the duel on its head. It righted the wrongs, squared the bets...then passed, leaving the music again to the stripped down sound of the pocket watch, and we return to the original standoff under totally different circumstances. Though Morricone's off-screen music had been winding up, in another contending duality, the drowned out watches had been slowly winding down.
A tear comes to Volonte's face like a miracle....
..."Bravo."
Watch the scene below...
